That is not true: the Latins did it like the Easterns at first; the current way we do it is a later development. Here is an excerpt from the Catholic Encyclopedia article on the Sign of the Cross:
At this period the manner of making it in the West seems to have been identical with that followed at present in the East, i.e. only three fingers were used, and the hand traveled from the right shoulder to the left. The point, it must be confessed, is not entirely clear and Thalhofer (Liturgik, I, 633) inclines to the opinion that in the passages of Belethus (xxxix), Sicardus (III, iv), Innocent III (De myst. Alt., II, xlvi), and Durandus (V, ii, 13), which are usually appealed to in proof of this, these authors have in mind the small cross made upon the forehead or external objects, in which the hand moves naturally from right to left, and not the big cross made from shoulder to shoulder. Still, a rubric in a manuscript copy of the York Missal clearly requires the priest when signing himself with the paten to touch the left shoulder after the right. Moreover it is at least clear from many pictures and sculptures that in the twelfth and thirteenth centuries the Greek practice of extending only three fingers was adhered to by many Latin Christians. Thus the compiler of the Ancren Riwle (about 1200) directs his nuns at "Deus in adjutorium" to make a little cross from above the forehead down to the breast with three fingers". However there can be little doubt that long before the close of the Middle Ages the large sign of the cross was more commonly made in the West with the open hand and that the bar of the cross was traced from left to right. In the "Myroure of our Ladye" (p. 80) the Bridgettine Nuns of Sion have a mystical reason given to them for the practice:
Icons from the ancient traditions showing Our Lord Jesus Christ as Authority,
Teacher or King, depicts him with thumb, index and middle fingers extended,
and the ring finger and sometimes also the smallest finger bent inwards toward his
palm.
This tradition did not make a lot of sense to me until I read about the
commentary a medical doctor made of the image found on the Shroud of Turin,
where the doctor explains that a nail penetrating the wrist in the place where
the Shroud has the wound showing would touch the median nerve of the hand,
and would therefore cause an uncontrollable clenching of the two smaller
fingers on the side furthest from the thumb during the crucifixion, as the
weight of the body pulls down on the hands. I figured this must have been the
image that St. John and Our Lady and St. Mary Magdalene saw while Our Lord
hung on the cross. And if that is all true, it makes perfect sense that this is
the basis for the Catholic tradition of a priest giving a blessing with the index
and middle finger extended. Also, if the thumb is also extended, that makes
three fingers, the number of persons in the Blessed Trinity.
I am only left wondering why so many icons show only the thumb and ring
fingers bent in, and the middle finger slightly bent, but effectively three
fingers extended straight: the index finger, the 5th or smallest finger, and
the thumb - the latter being touched on the tip by the bent ring finger. One
Byzantine priest told me this is meant to convey the act of "teaching." I am
left wondering if Our Lord may have bent his fingers thus while hanging on
the cross at the moment he had spoken certain words, such as "...Woman,
behold thy son... to the disciple: Behold thy mother" (Jn xix. 26-27).
Alternatively, perhaps Our Lord was wont to make particular gestures with his
hands as he spoke to his disciples, especially during those 40 days after the
Resurrection and before His Ascension into heaven. Or, perhaps even when He
appeared in the visions St. Paul had, for we have icons of St. Paul making these
same hand signs:
And Saint Luke:
There are others with various contemporaries, even St. John the Baptist:
When I read the words above, "At this period the manner of making it in the
West...," it isn't certain to me which three fingers they're talking about. For
any Boy Scout knows that the three fingers used for the Scout Oath are the
middle three, with the thumb used to hold the smallest finger in to nearly touch
the palm. That's "three fingers," but I highly doubt that's what the Catholic
Encyclopedia article was intending to describe; nonetheless, the article is not
certain to clarify this is NOT what it intends to communicate.
Nor is it too difficult to hold in the ring finger and the middle finger, leaving the
thumb, index finger and pinky extended: that makes "three fingers extended,"
does it not? Is that what the article is talking about? An uninformed reader
may think so. Because the article is VAGUE and AMBIGUOUS.
"And then ye bless you with the sygne of the holy crosse, to chase away the fiend with all his deceytes. For, as Chrysostome sayth, wherever the fiends see the signe of the crosse, they flye away, dreading it as a staffe that they are beaten withall. And in thys blessinge ye beginne with youre hande at the hedde downwarde, and then to the lefte side and byleve that our Lord Jesu Christe came down from the head, that is from the Father into erthe by his holy Incarnation, and from the erthe into the left syde, that is hell, by his bitter Passion, and from thence into his Father's righte syde by his glorious Ascension".
And with this, misspellings on CI reach a new 'low' - even if it is historically accurate!