Not talking about the low resting HR improving by exercise.

I was under the impression that the max HR is generally very rigid with slight decreases occurring as age takes its toll. A few weeks back, my max HR did a step-function like change, and has been stable at the new point since then, it was large enough to shift the numbers that I use to limit how much high intensity sprinting I do.. It was about 175, and now its a good 183-185. Resting has remained at 55 or a bit lower. I don't really mind, as maybe I can pretend to be about 5 years younger??? (i'll be 48 soon)... 1 min recovery remains a respectable 35-40bpm.

Power and endurance didn't seem to change noticeably, just the numbers that I see on the HR monitor's output at different exertion levels. (and no, monitor didn't go nuts, compared it with other monitors, and its fine).

Is this odd, or am I mistaken about maxHR in the general population? I was just really caught by surprise by the change.