I think you might have misunderstood me. 1 combat-round = 1 HP's worth of combat, not a complete individual "Tank vs. Rifle" fight to the finish.
"Total Tank HP" is for all Tanks used (vs all Rifles attacked), not per Tank.
(It also seemed more sensible to
show percentages at 1% increments, since I will rarely if ever be bringing >1000 HPs to a battle!)
Point is, a stack of 5 vTanks has 20 HP, so against 5 vRifle city-garrisons*, I can expect to have lost about 38% of my Tank-HPs (i.e. 8/20 HP) at the end of the first full set of 5 battles.
But how that 8 lost HP is distributed across my Tank-stack will still be essentially random: (barring promotions) I might have 5 Tanks at 4/4, 3/4, 3/4, 1/4, 1/4 (= no units lost = 'best' outcome?), 1 Tank lost and 4 Tanks each at 3/4 (and more vulnerable to Cav-attacks!), or I might have 2 Tanks lost and 3 Tanks all at 4/4.
*NB
@ArenE: This is a theoretical example: apart from the capital and/or Wonder-towns, most AI-cities won't have more than 2-3 dedicated defender-units.
That said, the AI will
also build Rifles for attack (both AI-strategies are flagged for this unit in the Editor) if it has Nationalism but lacks MilTrad and/or Horses. And injured 'attacker-Rifles' will retreat to the nearest friendly town to heal, so will add to any 'defender-Rifle' pool that's already there.
That's why, once you're going on the offensive, it's generally better to focus your forces on taking the next target-town
first, and only do mop-up on AI-units in the field afterwards if you still have bombardments/ attack-units available and in a fit state to do so.