TarzanoStyle
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The heroes are generated due to the AI mod; this feature is enabled/disabled in the map selection menu. As for the army strength adjusting, I don't know; randomness should be at play there, so it's either due to the heroes, you're just unlucky, or there's some kind of script (but then these scripts should be enabled on your maps, so I think this is the least likely option).
Ideally, you should run a test: attack an object with a weak hero and a weak army, retreat from the battlefield, and then attack again with your strongest hero. If the army is the same, it's just randomness; if the army has changed, it's a script.
That's the thing, when you attack with a scout, there's a standard unit composition, but when you attack with your main army, there might not be 120 gargoyles with 850 hit points. But after the victory, in addition to the standard resources, the army also joins you (gargoyles join gargoyles, harpooners join gnomes, etc.), but most of the time, these battles aren't worth it.
Oh, and the buttons at the start of the map (guards) are not active, meaning you can't uncheck the box.