[PvK]

I'm not familiar with a bug where the estimated chance to hit in tactical combat is actually wrong.

In general though, the reason for this is that you don't have all the boni that your opponent has:

* Ship and Fleet training
* Cultural Space Combat modifiers
* Racial Aptititude Offence and Defence modifiers
* Vehicle size modifiers
* Per-weapon to-hit modifiers (not always shown)
* Talisman device
* Scattering Armor
* Stealth Armor
* ECM
* Combat Sensors
* Temporal Event Predictor facilities
* Neural Network devices

PvK