Let's be fair here, if there was MMO video game realism where levels mattered, the game would have involved Orion grinding boars for a long, long time and then when he gets a power-up he'd be a filthy cheater. And Penelope would just kinda get one-shot at the start of everything. Plus it'd be super unrealistic that they're involved in the plot at all despite not grinding the thing 24/7.
My headcannon is it's like a shooter where levels just give you a perk here and there, while weapons are key and your basic weapon is good enough to kill stuff while big weapons are an edge for experienced players to wield properly (you'll get one shot by a shotgun to the face either way, but you can get a shotgun with a bit more range or capacity). And Orion just gets less lag than everyone else. Don't even remember if it was ever explained, it's just what I automatically think of.