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The workout zones and the formula 220-Age are wholly arbitrary and are as incorrect for any one individual as they are correct for another. As an example, I have been able to go out for a ten-mile run where I maintained an average BPM within a single beat per minute of my theoretical "max" heart rate.
I've been wearing a Fitbit device with heart rate monitoring of one model or another for nearly six years now; one thing which has proven true, at least for me, is keeping the band tight enough to prevent the tracker from moving around on my arm. The two most common circumstances where I see wild swings in tracked heart rate are real early on in a run, some times spiking 60+ bpm over my average for the rest of the run; the second place where I see obvious errors show up later in runs in warmer weather when my skin is thoroughly saturated with sweat, allowing my tracker to move around. A side comment on that last bit, I'm old and start slow, often warming up in the nine to ten-minute pace zone, and yet finishing the last three or four miles of a ten-mile run down in the low eight minute to high seven minute pace; needless to say, running at that speed means more rapid and aggressive arm movement, and combined with the sweat leads to movement of the tracker on my wrist.
