Tactical Battles vs Automatic : Combat Math Question

I'll keep this short because I don't want to write a wall of text, and I would really like an answer to the question before people reading this post get bored and move on. 

Basically, is auto-combat working correctly?

I love tactical combat, and I love the tactical combat feature.  I'll even use it when I vastly overpower an opposing army.  So I know how to use it, and I know how to position units to inflict maxium damage, etc.

Now I also know that archery isn't modeled accurately with auto-combat, which makes sense.  I normally don't research archery anyway, so my armies almost never have archers.

But here's the situation I'm running into.  If I have a stack of lets say 20HP, 20ST, 20DEF and I'm attacking an enemy with the exact same stats I would expect both sides to give and take about the same amount of damage.  But what happens, is that I'll hit the other unit for 1 or 2 points of damage, and they'll hit me for 4 to 6.  Since my stack wears down faster, I start doing less damage and will start missing on my attacks more and more.

I just figured they had a different type of armor or something, no big deal.

But when I do an 'auto resolve' on the battle, I'll win by a landslide.  My stack will take maybe 2 points of damage while wiping out their entire 20 HP.  Is anyone else seeing this happen?  When I look at the combat log, I'm hitting the stack for damage that I've never seen before while playing tactical.  What's up with this?  When I close my eyes, does my army change into a bunch of bloodthirsty killers or what?