errr..no. you thought wrong.
this is one of the stupidest things ive heard of. imagine stacking a bunch of 10lb on a military press on one side, and teh same number of "5s" on the other. 5 being 5 kg and 10 being 10lb. lets say you have 5 per side...
and so on.
stamping the weights a lb more or less here and there is well,..theres a thread already at GB about stupid americans.
the difference for one plate is 1 pound if they do weigh different and not just a name
5 5kg plates = 25kg or 55lbs
5 10lbs plates = 50lbs or 22.7kg
difference isn't huge, so why not just "rename" them to closest possible number.....
would you notice a difference picking up a 45 and then a 20kg? a 0.1lbs/0.4kg difference...
I'm not saying this is what they are doing, i'm saying it's a possibility and no one would be the wiser.