I thinks its three things really. First part is, it was introduced pre skip. And like almost everything pre skip vs post skip, Oda retconned it. Remember when Oda said 400mill is exceedingly hard to surpass and go up once you hit it? And Ace was 150mill above that, he’ll tha WG considered him a bigger threat than Doffy when Doffy joined tha Warlords. And that’s with tha WG knowing both of their histories and such
Secondly, Ace spent a year doing nothing noteworthy. He wasted a year chasing Teach. And even before that wasn’t it stated that tha WhiteBeard Pirates hadn’t really made any moves in years at that point? So he hadn’t really gotten a chance to go and jus do crazy shit to boost his bounty.
Lastly, he was young. He got 550mill in 3 years. Cracker has 800 mill and he’s a good bit older than Ace I think. Perospero is much, much older than Ace, wit decades of pirating and only got 150mill on him. Same wit Katakuri, hes been pirating for decades. And Ace got half his bounty in like 2 years( he spent a year searching for Teach so his bounty ain’t move in a year)