I looked but couldn't find this mentioned already (it has to be, and I just missed it, but...)

Strength is listed as 5 in the Attack value determination, but only 3 in the Unit Details part of the UI.
I can't think of anything (items, obviously no spells as they're not implemented, etc.) to explain the difference. I may have applied a Strength Book to this char and it may have changed one but not the other.