If it works this way, then it is indeed FUBAR. This means, as one example, that a modern paratrooper (6-11-1) attacking a spearman (1-2-1) on a mountain (def bounus 100%) only has a 6/6+4=60% chance of winning. The Bronze Age spearman has a 40% chance to win...
Another example: A berserk 6-2-1 attacking a modern armour (24-16-3) in the open, let us say on plains, has a 6/6+16=27% chance of winning. If it attacks a tank (16-8-0), the berserk has a 6/6+8=43% chance of victory ie more or less a 50-50 chance.
If this isn't FUBAR...![]()
It does. Your examples are mostly correct the only modification is that the chances you are giving are single round chances. When both units have 4 hps for example, then the stronger units chances of winning the overall battle are greater than those figures, but not high enough to make you happy, Im sure

To me, it is simply about gameplay over realism. I dont want the player with the research edge to win almost all the time. I think it makes for a more interesting game if the player with the backwards units can use tactics and terrain at least to make the more technologically advanced player struggle some. Think of it as the game punishing the strong and trying to challenge you

I agree that there are plenty of things they could have done differently and possibly better, but Im not sure that the way you thought it worked would have been an improvement.
Judging from the graphics/animations and when damage occurrs:
1. Defender free round -> 50% chance to damage x Defender's A/Attacker's D
2. Attacker's round -> Attacker's A + rng vs defender's D + bonuses + rng, the greater score wins the round
3. Defender's A +rng vs Attacker's D + bonuses + rng, the greater score wins the round
4. A
5. D
(etc til one party is "killed" or flees)
Ie. the combat system used in AD&D PC-games and early Military games.
Correct me if Im misinterpreting this, but it seems to me that units having separate attack and defense values would be almost irrelevant under your system. The attacking unit is forced to use its defense value, and the defensive unit uses its attack value. As an example, if a 4/1/1 unit attacks a 4/1/1 unit, who should win? If there is to be any significance to separate attack/defense numbers, then the attacking unit should win that battle most of the time, but I dont think your method would give that result.