The Apple Watch does seem to keep recording heart rates from the watch sensor every few minutes (as opposed to every few seconds), just as it does for the rest of the day, so the two different sources could explain it. There is nothing the app can do about that though.I have another theory: could it be because readings from the HRM and from the AWU itself on the wrist are combined/merged?
I am connecting my Polar H10 HRM directly to WOD and have noticed that sometimes, after passing HealthKit, the heart rates in the workout become a combination of the readings of both the Polar H10 (via WOD) and my AWU (via Apple Health). This often is unnoticeable as the value measured are close. However, my AWU regularly starts measuring my HR erroneously at 180bpm in the first few hundred meters until it drops to a correct level (which is why I use the Polar H10 btw). On shorter runs, this becomes visible in the workout (and also blows up the average HR).
NB I have seen this strange 180bpm starting behaviour with my previous AW7 as well as with the AWU.
If you get these sort of problems then it is probably best to connect HR sensors via watchOS instead of directly to WOD.