That's a great guess honestly, better than I could come up with, but I still have a few others:
1) Others seem to work in groups, they don't go solo, so the exp is split.
2) Motivation plays a role. Mid-class heroes seem to spend a lot of time banging, and low-class heroes seem to be way more careful. He has the right amount of daring and only bangs in his free time. That's a big difference.
3) It could be equipment too. Low-class heroes may not have enough money to get good equipment yet, while mid-class heroes were mentioned as acting arrogant and treating sellers like shit, which might have led to them being given worse recommendations than the MC.
I think it's a combination of these factors + the fairy guide.
Even a 10% difference in efficiency can compound quite quickly to a wide gap. If I didn't mess up my calculations (which I probably did), assuming the more experience gained the more experience you can gain (levels, skills, and stats translating to more and stronger mobs killed), at 10% per day there's a 61% difference in experience earned after 5 days.