CathInfo.com

In the pre62 system the 20th is the fourth Sunday.  In the Breviary the first Sunday of the months August to November was the Sunday nearest the Kalends of the month, i.e. the first day. 

So, this year, the first Sunday of September was actually 30th August (Pentecost XIV); the second Sunday 6th September (Pentecost XV); the third Sunday 13th September (Pentecost XVI).  Following the changes introduced in Rubricarum Instructum the system changed and the first Sunday of the months August to November was always the first Sunday to fall on, after, the Kalends.

The other way to work out the September Ember Days is to remember they are the Wednesday, Friday and Saturday after the feast of the Exaltation of the Cross. That was on Monday of last week so, traditionally, the Ember Days followed.  However, if using the 1962 rubrics the Ember Days will be the first full week after the Exaltation as 'Pax Vobis' explained above.

The rule is two-fold:  1) after the feast of the holy cross.  2) the third  FULL week. 

Last week was the 3rd week of Sept but not the 3rd FULL week.  A full week has to start with Sunday. 

The 62 missal changes were approved by Pius XII so technically ok.  (As far as the calendar goes).  I can see why the SVs go with last week because they reject the 62.  Not good or bad, it’s just an explanation for the difference.