I thought I'd reached a new maximum HR a few months back. However it turned out that I'd brought my VDO
altimeter with me rather than the VDO HRM (they look very similar).
It turns out that the topography the climb on my training loop coincides remarkably well with my expected HRM readings, that is until it goes over 196 (my max HR) and climbs up to 230m. Eventually I twigged that this was altitude and I hadn't beaten my max HR by 34bpm.