In 100% of the games there's 4 worms on both sides with equal health
Yeah, for the first 30 seconds usually.
You gotta count in different damage through weapons (Dynamite > Bazooka > Firepunch) and most importantly failed turns/attacks. No matter who's failing an attack, there's gonna be your difference in health. And it happens frequently even in an easy scheme like Shopper. You won't ever have a game with completely leveled health on all worms, that's why piling is important.
I'd very (very) likely win a Shopper game by piling every turn against someone who doesn't pile but only hides.
Anyway this went slightly off-topic. Shopper with AFR as a basic rule, why is that? I still have no idea.