Could be a few reasons, there is a thing called the "critical hit bonus" which i guess it doesn't show on mobile version it's a random amount of damage dealt. Also as that person has more mobsters they're able to bring more items to the fight.
i never understood that either. Sometimes I lose against an opponent that is only stronger than me in 1 area. Like defense. Then other times I will win against someone that is only stronger than me in defense. So all guesses is as good as any. No matter what. You need to be stronger in everything so there is no questions left unanswered about why you would or would not win. A strong personal attack must be combined with strong items and mob to hold those items. A player who has all their mob without them being equipt with strong items will lose against a player wiith a few mob where all of them are equipt with the best items available.