NOt saying that something isn't off with the numbers of the OP's group but fwiw my level 8 half orc berserker barbarian/fighter who wields a flametongue axe (+2d6 fire dmg on a hit) easily deals between 70 -100 dmg per round while raging and crits on a 19-20 (add in half orc cit dmg dice). Add Action surge into one of those rounds, and reckless attack for advantage on all rolls ... well, the berserker barbarian can be totally mental. But exhaustion limits how often you can be that epic, so really 2/day is really all you will ever be that crazy deadly. And then you will need some serious rest. If you are using frenzy 3/day you will have 3 levels of exhaustion at the end of it ... bad news!A level 8 frenzied barbarian with GWM and Str20 will do 2d6+17 per attack.
Since missing once is more likely than critting once, I'd be surprised if it reaches 72 damage per turn.
You might have rounded up his best round, but considering you managed to get expertise in athletics I'm not surprised the numbers don't match reality. You really are deep in la la land.
NB: the aforementioned character uses the 27 point buy system