I guess the main reason is not the classmate (the top uni in US also have many top student)
but the grade curve instead...
it is all about grade inflation.
and I bet that the whole academics knows about this and won't simply compare the number, and will take the university and location into account too.
e.g. http://www.foreigncredits.com/Resources/Grade-Conversion/
(imagine that they are not looking at a HK one will ABCD, but a France one in 20 point scale, or a German 6 point scale with 1 being the best or a UK with scores and honors...........)