It's actually a bugged design.

First you should know that every member of your longbow stack does damage separately and then results are combined to single hit.

So it's 0-6attack roll of archers (if they have attack of 6 per unit) vs 0-5 defense roll for ogres (if there are 3 ogres with defense of 5). Repeated as many time as you have archers. That's good enough to do decent damage, especially if you have more troops.

Now, where bug kicks in:

When unit starts to drop in size, defense rating of full unit doesn't decrease, but combat calculations assume it does. So when only 1 ogre is alive, game would give it one defense roll of 0-15 (total of 15 divided by 1, compared to total of 15 divided by 3, when all 3 are alive), instead proper 0-5, which makes it very difficult to hit with those archers, since their base attack per unit is pretty low.

At least, that's how I understand it works.

P.S.

Btw, same bug happens on attack too.

And I would be curious to see the save.