That is amazingly flat ! I will have to examine my setup for possible tilt with travel direction change during probing.
I use the Orion piezo sensor so my sensing happens right at the nozzle but I use a CR10 3 V roller carriage and the pizo sensor requires a sort of loose nozzle. There are a number of possible sources of oddities.
I have a second CR10 printer that uses a BlTouch and I have always seen these ripples but could never track them down. This gives me a whole new dimension to check out so thanks for bringing up the issue and thanks DC42!