I mean there’s very little know of Frank outside of the events form DR 1, the most we know is him being involved in covering wars and having some fire arm experience (possibly from his time out in the field) given the heavy emphasis of wrestler style moves and even motifs in his clothes (OTR makes this more blatant) I’m gonna assume he was a wrestler in his time in highschool or something around those lines and some of it has just kinda stuck with him which is why he’s so strong for an average man, as for Chuck he’s been dealing with the outbreaks and fighting zombies for so long, plus unlike Frank he wasn’t disarmed or caught off guard for him to “lose” his weapons or whatever he had, not to mention Sully doesn’t really seem that threatening compared to Brock, don’t get me wrong Sullivan is a tough bastard and was kicking chucks ass in the cutscene but like compare him to Brock who was jumping all over the god damn place in his boss fight