Joined: Aug 2009
Posts: 6,768
Likes: 5,405
From: Minneapolis
Bikes: 2022 Salsa Beargrease Carbon Deore 11, 2020 Salsa Warbird GRX 600, 2020 Canyon Ultimate CF SLX disc 9.0 Di2, 2020 Catrike Eola, 2016 Masi cxgr, 2011, Felt F3 Ltd, 2010 Trek 2.1, 2009 KHS Flite 220
As already said, HR depends on lots of factors. It's affected by heat, by your hydration state, but your accumulated fatigue, by your fitness.
Personally, and anecdotally, I seem to have two different HR regimes. When I'm not at my best fitness, my HR is higher for a given effort - of course. But also, this is when I can experience my maximum HR. When I am more fit, my body seems to switch into a low HR mode- I can complete a very similar ride at lower HR, but also, it's harder for me to reach my maximum HR.
Unless you have some other reason to suspect something is going on, don't over-think this one.
The variability of HR is one of the reasons that many people base their training on power, rather than HR.