After some time I have decided to calculate my Damage Modifier of the drone.
The drone I have calculated damage for is Imperial Nave Praetor.
I have next skills: Heavy Drone Operation 4; Drone Interfacing 3; Gallente Battleship 4;
According my calculation I should have next result: 1.68 * 1.2 * 1.3 * 1.4=3.66. In pyfa, I am getting 3.6 as well.
However, in the game it shows 3.4.
It looks like after daily maintenance it is refreshed and shows correct value, according to my calculation.
I think there was some delay to refresh the status. The reason I am saying it is that only yesterday, I got new level of Gallente Battleship. So, now it is OK.
In some cases the order you apply bonuses in can and will make a difference especially once you get into modules with stacking penalties so watch that order. It has been a really long time since I looked into this, however if memory serves me well here is the order. Someone please correct me if I am wrong.
When comparing results from different sources rounding errors and other math related things can change the end values slightly. Another factor is the sometimes the math is based on an optimal situation, while other base in on a worse case, and yet others try to use a middle or the road approach.