There are a lot of things at work, including the terrain which can affect unit strength and hitpoints.
Also remember that there is likely an attack cycle for each group of units, and all the groups will return fire using the defensive stats each and every time they are attacked. This can lead to additional damage done per combat cycle.
From your description you were attacked, and your tanks defended, and then you counter attacked and the AT defended. Seems like a fairly normal exchange to me.
You should read the pinned post about the X factor, and in particular pay attention to the math equations posted by game designer Freezy.
