MHR is most likely very activity and mental state dependent.

I "retired" to bike riding from rock climbing as a primary activity. (66 YO)

I can't get even close to the MHR that even a relatively easy climb will induce on the bike.

Another thing that may be at play is that a really good indicator of fitness is HR variability (how fast your HR responds to workload change)

That might be pushing your max up as a combination of being in better shape and a competitive mindset.