Oh boy, nobody understands me 
Stock A500 and stock A600, noooo FAST RAM, give the result of 0.75 compared to stock A2000.
Why ?
All those Amigas have the same 68000 7MHz CPU.
Do stock A2000s come with FAST RAM ? That would explain the difference in speed. If not, then how is it faster with the same CPU and CHIP RAM alone.
It's a comparision against an A2000 with FAST RAM (which was the most common situation), as you mentioned if you add FAST RAM to an A500 it reads the same.