What jumps out at me, is that it says there are no rubrics for the postures of the laity at High Mass, and that for Low Mass, though there are general rubrics, they are not preceptive (precepts of the Church, like that you have to go to mass on Sundays and Holy Days, communion at least once a year...) they are only directive. That means it can be changed, and that the postures come from tradition and the customs of the country prevail.
Let's discuss the simplest mass first, the Low Mass. Notice that the custom of the USA differs only in that Americans sit after the Offertory rather than the instruction in the (Rubricre Generales Missalis, Tit. 17, n. 2.) De Herdt (Vol.I, n. 146) where it instructs to kneel and remain so throughout the Mass, except during the two Gospels.
Does this mean that a priest can change the postures in his USA chapel to conform with Rubricre Generales Missalis, Tit. 17, n. 2.) De Herdt (Vol.I, n. 146)? The answer is no, simply because it is not the custom of the country. Moreover, it just makes sense, since if each priest were to make his own changes, it would cause anarchy in the pews from all the visitors and those that have not caught on to the change or just do not want to change.