Different drives use different chips and different controllers for those chips. Not all flash drives are created the same. You also didn't mention how the drives are formatted, which could potentially affect performance as well.
As for the speeds copying back to your computer, that is probably governed more by the USB ports. Most likely, those flash drives all have higher read speeds than your USB ports support, so they all top out at the same speed. Typically, in any kind of flash memory, read speeds are higher than write speeds.