Steve -
This is actually a suspected v3.11 bug, and I don't know if you've changed ground combat since then, so it may or may not still be valid.
Simply put, for some time I've been getting odd results out of ground combat, where it appears that out-numberred forces seem to be doing a lot of damage against numerically superior forces. I've written this off as being atypical but statistically possible results, but I've just had something happen that is so unlikely that I felt I had to mention it.
The situation is as follows -
Side A: 4xHeavy Assault Division (HAD), 3xAssault Infantry Divisions (AID), Offensive strength 343, Defensive strength 282
Side B: 6xEngineer divisions, Offensive Strength 0, Defensive strength 96
This is quite obviously a lopsided match, and Side B should have been destroyed quickly. That is not how it worked out, though. After the first round of combat was resolved, Side A lost 4xHAD and 1xAID, while Side B lost 3xEng. This result was so far from what I expected that I examined the Event Updates screen to determine how this could happen, and this is what I discovered: Aurora showed Side B as having an attack strength of 1875 and a defense strength of 339.8, while Side A had an attack strength of 1958 and defense strength of 223.
Both side's offensive strength was drastically overstated, particularly Side B's (which should have been 0 and wasn't attacking, only defending), while Side A's defense strength was understated and Side B's was overstated.
I don't know what modifiers Aurora uses for combat, but in any case, Side B had no offensive strength at all, so how it could go from 0 to 1875 I have no idea.
Kurt