The Zowie mice I own (ec2 and fk) both have higher CPI than their rated value. This appears to be a common experience among owners of these mice.

I have an FK, and as povohat suggested the DPI steps are incorrectly listed.
I measured it on mine a while back:

450dpi is actually 515
and 2300 is 2565.
I didn't measure the 1150 setting. ( or whatever it is )

You can test this for yourself by getting a sheet of paper, ruling a 5cm line on it, and setting m_cpi to the value you've set cpi to on the mouse, then set sens to 72. Keep testing it on the paper, altering m_cpi until you find a measurement that accurately reflects the 5cm/360 sensitivity.
That's how I tested.
Let me know if your measurements are the same as mine.

I emailed zowie and never got a response.
Aside from this flaw, the mouse is absolutely perfect.

yeah thank you so much, i did the same test, its 480 DPI

puretrak talent btw.

there is custom lens in zowie am and fk for lower lod
may be this lense changes dpi a bit? this lense can be removed.

its like 28-29 cm instead of 30.8

What is your problem exactly?

edit: Ok, you just asked what causes this. Here is the answer: CPI depends on a surface. Have a nice day.