If the distance between the two is almost identical but the average speed is not, that must mean they have a difference in time. Have you compared those? Does the app have an auto start stop functionality?
I used to ride with a bike computer with sensor though wireless on the wheel not wired and wahoo on phone. When I didn't stop, both time and distance matched close enough for my liking. When I did the wahoo always read less time given the auto stop is what 3kmh or maybe 5 idk what I set. Instantaneous speed displayed differed too though in most cases was fairly close and the more even i kept going the more aligned they were. Similar to my two HR monitors. My polar band reacts a lot faster than my fitbit watch, so one might be 150 the other still 140, but a few seconds later the fitbit would catch up. Similar going down and overall the average HR was never more than 1, a few times 2 bpm different.