Speed differences in DDR RAM?

I get rather confused when people say things like "PC3500 OCZ is faster than PC3500 Corsair" or "Mushkin is slower than Geil". Give the same speed rating and chip manufacturer (Winbond), why would one be faster than the other? The funny thing is there *is* indeed a speed difference judging from a number of review sites.

Can someone care to explain this to me?

certain memory manufactures rate the same quality chip at different levels

corsair might bin a chip that can run pc3700 fine and rate it pc3500 so they can guarantee it runs pc3500 at cas2