It's possible this is just the extreme end of the variance, but a single lvl 2 artillery vs 2:AC2 1:LT1, 2:INF2 and 1:MOTI:1 took zero damage after both an offensive and defense round.

Update. By the time my artillery were ready to fire the Canadian infantry unit had reached the melee battle. After 2 of my artillery fired, the infantry was at around 15% but the Canadian artillery was still at 100%.

When that stack of 4 which shows the AA (it had 3 inf) reached melee range it killed the Canadian infantry and reduced the artillery to 20%.

We all see stuff like this on a regular basis, but is this the expected type of variance or are they glitches?

The Canadian artillery was in front of the Canadian infantry and not in the initial battle. When the infantry caught up and joined the melee, I believe they joined into a single stack, but even if they didn't my artillery would have targeted them both, as if they were in the same stack. So anyway you look at it the solo artillery survived a melee round (look at images: both sides attacked/defended so it's equivalent to two rounds) against a descent sized melee stack with no HP loss, followed by an artillery round where it was grouped with an infantry with no loss of HP. The artillery round is understandable, but the melee round(s) is extreme.