Smilingcat describes it very well! At least, from my own observations. My HRM occasionally registers 240, at which point I know there was some glitch (maybe interference from somebody else, but maybe just some weirdness within the unit). I also don't really go by the calculations - they're just a ballpark, and if you are over 220 - your age, good for you, and don't worry that you're going to explode!

Not sure what my absolute max is, but it did register 189 last Saturday in my 1/2 marathon. Generally after a running race it shows 186 or so. I don't do many short, hard races these days - more 10 miles+, where I don't really max it out. I'm also old & wimpy.

Last summer, after I started noticing my hrm recording in the low 30s when I was trying to find my resting rate, I mentioned it to my doctor. She had me wear a Holter monitor for 24 hours. My lowest reading on that thing was 36, probably while I was sleeping. I was pretty pleased about that.

I just turned 42. I haven't seen much change in my max over the past 15 years, but I also didn't have an hrm back then. I was in better shape (and weighed less), though, so I'm sure things have changed.

220 sounds pretty high to me, but that doesn't make it impossible - you may just be a goddess, able to pump far more blood and oxygen than I can. I'm kinda jealous.