As I remember researching the difference I think the CE rating is 50 and the USCG is 55. I think that refers to percentage of flotation compared to the average weight of sized (S,M,L,XL) person. So the USCG would be 10% bigger, that seems like quite a bit to me, and it feels like it.
50 refers to newtons. 50 newtons is the smallest buoyancy aid you can get with a CE marking. Frankly this is probably not enough for long distance racers, but fine for around the buoys when a safety boat is in attendance. Hence the name "buoyancy aid".
The smallest adult US PFD, type I, is 22 lbs, aka 98 newtons. This has almost twice the buoyancy of a CE buoyancy aid and is pretty damn close to a 100 newtons CE Life Jacket. Note, those are no longer called buoyancy aid, but life jacket.
