So, what can we tell, n900 aren't made equal, but there imho shouldn't be so noticeable gap between some.

How to check were my tablet was made? Should I go through papers? I know it's designed in Finland, hehe

But now, how it should be?

The n900, should go over 900mhz with smart reflex or they should not? What can we describe as "golden unit"? Is it even possible to describe?

And finally? What are those variance you mentioned?
Is it better circuit capacity? What I mean is, are those "better" units better handle higher voltages and frequency? Sorry for my unsettled describe, Im not an hardware engineer :P