I wonder how many people actually realise how mice work.
Just because two different manufacturers say that their mouse is 800 DPI, does not necessarily mean that they both measure exactly 800 dots per inch. The chances are relatively high (yo pun) that each mouse is only "close to" that value, but could fall either side of it, and could still be true for mice from the same manufacturer.
This is easy to test when using the sensitivity/360 degree calculation. Then... in an FPS game with raw input, and a degree/major-axis HUD on screen, actually measuring the distance on your mouse pad with a ruler. The difference can be quite significant and it is sure to be different between mice. It's also best to test on different surfaces, since different mice behave differently on different surfaces... another factor.
Afaik one of the Zowie mice had this issue with its lens, where it's 450 DPI setting was actually 490 DPI.
So in short... it is for you own good to find a good mouse that you like, and stick with it for years... When you change mice, something else changes too, besides the grip, and it's not always as simple as a relative DPI value, as I have experienced. If you came from a bad mouse, adjusting to a good mouse may even feel weird.