Why is the rating for the single handed F16 rating (on a returns based system) quicker in the US?]
1-up is not rated faster; 1-up and 2-up are now equal. The 2006 handicap tables had the 1-up configuration a good percentage slower (65.2 to 67.1, I think). No data was being collected at the 67.1 handicap number because sailors lobbied to sail at the more accurate 65.2 handicap (2-up rating) - actually they were embarassed to sail at 67.1 because it was a truly gift rating.
I just looked at the tables on the US Sailing site and there is now only one entry for "Formula 16" and it is 65.2. Officially, there is no "Formula 16 1-up" handicap number. The 2008 numbers will be released in Spring of 2008 and the predication is that the 65.2 will be closer to the F18 rating.
It is important to note that US Sailing dropped the F16 1-up rating at the request of the US F16 Class Association. The topic was dicussed openly and the majority was for aligning with F16 class rules where F16 1-up and F16 2-up race on elapsed time - even in a Handicap/Open Fleet.
If you do not like it, do not blame US Sailing. Blame the USF16 Class Association. Most F16 sailors (me included) are in agreement that it was the right thing to do.
It doesn't make sense to race on elapsed time at the Gulfport F16 Invitational, then go to The Mug Race (large Open event) and not race on elapsed time. That was the thinking, at least.