An example of combat ineffective rates in real combat

[Imp]

Generally infantry toughness in the game seems random

Not completely, yes their are a lot of random factors but experience plays a big part.
Try playing with the same units one side set to 65 experience & the other 120, actually that wont work they will be more accurate to
Buy the above & switch weapons off or modify so cant fire back. now use them as target practice & you will discover the experienced guys avoid more incoming.