Why does +xp shows when upgrading unit?
Every time I upgrade a unit +xp shows where is some number. What does this mean? It certainly does not get added to the units xp.
It's a minor UI bug due to how units are upgraded. The new unit is created, the old unit's properties are copied over, and the old unit is destroyed. Thus, the game sees XP being added to the new unit and reports it in the UI.