That was very likely just spurious data that Fitbit discarded as such. Optical HR sensors can generate a lot of false data like this so smoothing filters are used to discard many of those bad readings. Even then, some still slip through. Fitbit and cardiogram bothmlikely use different filtering algorithms; it wouldn't surprise me at all that the spurious readings that do slip through are in different locations between two different apps.
I read an article that I believe was done by DCRainmaker where he compared raw optical HR sensors data to the much more accurate chest strap-style HR sensors. The raw data from the optical sensors was an almost indecipherable mess compared to the raw data from the chest straps. However, once some smoothing filters were applied, the data from the optical sensors was a much better match to the data from the chest straps with only an occasional spurious reading here and there.
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