If you're a summoner, the difference comes from one of them calculates bio and bio II, the values given from these spells are inexact due to them not having an initial damage hit.
Otherwise, the issue is the fact that other people have to be relatively close by for their information to be read- so not having everyone in your party close to you at all times will throw the results in your favor.

AND one other question.. are we not supposed to use apps eventho they don't modify any files within the game itself to read our dps and such?
Technically, interacting with the exceutable isn't allowed either, those apps do. So its quite possible that they're not OK. Really, the best course of action would be to wait until the add-on API is released.