The difference is Mahomes and Burrow are consistent. It's a fact. They are both good QBs that have great games sprinkled in. Allen tends to be either a great game or an average game (sometimes even a bad game).
Mahomes 3 best ratings: 144.2, 132.4, 129.6
3 worst: 85.2, 80.9, 78.5 (week 3 vs the colts)
Burrow 3 best: 138.2, 126.6, 126.0
3 worst: 85.3 82.7, 61.7 (week 1 vs the steelers)
Allen 3 best: 134.1, 128/9, 119.2
worst: 71.3, 68.4, 46.8(week 9 vs the jets)
They do have less bad games than Allen and that is the narrative. I don't care for the stats about it because what he does as a football player can make up for what he's not doing as a QB but that's where it comes from.