Well, let's see:
Lancers charing Iberian Infantry: 18 soliders killed
Round Shield Cavalry charing Iberian Infantry: 12 soliders killed
Lancers charing Chosen Swordsmen: 10 soliders killed
Round Shield Cavalry charing Chosen Swordsmen: 3 soliders killed
Yep, it works for me...
EDIT:
It's pretty possible that the reson why you see the difference is that you need to understans that attack is not a lethality stat but stat of dealing blow. So if you take a unit with already high attack against unit with very low defense, it will by itself deal maximum lethality (killing maximum number possible in one blow), making extra points of attack (by charge) meaningless.
You see that in my example, that against low defense unit charge added 50% more kills, while against high defense unit it tripled it.
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