I tested mine this morning and I observe the same thing. When I have a line rigged, my probe reads about .5 mph faster than when I don’t.
I have a blacks release with the probe directly below. When I Pop the release, my probe reading drops by.5 mph. Was out with a charter on Ontario yesterday who had a blacks release and a cannon release off the ball on his probe rigger. When I asked him why, he told me that having a line in the blacks screws up his speed reading.