This is not what's going on for you, but misery loves company and sometimes birds of a feather... In Garminland there's a 24 hour HR chart, with your movement level and recorded activities superimposed over it. That info comes from your all day data, which they seem to store in 5 minute increments. So my daily max HR is always reported lower than what it actually hit going up the short but punchy hills around here.
What I would do personally if I really wanted to know is open the fit file in Golden Cheetah to get the raw data, bring it into Excel or SQL, and then try to come up with their numbers.