DPI Analyzer Edge to Edge vs Ruler

[Eikou]

Hey for some reason I'm getting very different results with this test. Its of course not a big deal but lets take my current mouse (Model o)

If I measure 10cm @ 400 dpi with a ruler I get 370 DPI every time. 

If I use a piece of the same make of mouse mat cut to precisely 10cm and track the sensor from the very edge to the very edge I get bang on 400DPI. 

 

Its obviously quite a large difference and its not hugely important but does anyone have any idea why this would be and which do you think would be the most accurate. I'm leaning toward going edge to edge. 

 

Many thanks