Also, maybe wifi assist is turned on? If so, and the os determines wifi is slow, maybe doing some work over cellular? No speed tests, but, have noticed on somewhat slow public wifi but showing all wifi bars, the cellular data counter has ticked up, so, have turned that off until really needed.
And will echo above, have seen wild swings with back to back speed tests. Would get a better idea by running multiple speed test in each mode, to get an average.