How does DPI effect sensitivity?

[Vallium]

Just curious because I was using my G400 on Halo MCC with 40 sensitivity.
When I plugged in my G500 at 4000dpi vs the G400's 3600dpi it felt much slower. Had to double the sensitivity to get a similar feel.
I was under the impression that higher DPI = higher sensitivity but it does not seem so in this case.

[OBsIV]

When you run your G500 at 4000dpi, it's emulating a lower DPI sensor (an "uneven" down-scaling from 5600 to 4000). So, I'm not sure how accurate of a comparison it is.