Strange! How about an experiment: next time you're on a buoy or anchored and in a strongish current check you're getting this low reading then start the engine and just take up the slack on the mooring or anchor and see if the speed is any different.
I can't see why there should be any difference.
You could also check the current with a Dutchman's log to make sure it's really as strong as you think it is.
-------------------- Danny
TidePlan English Channel tidal passage planning software