Powerinspect giving weird values?

Hi guys and gals, about to start bashing things with a deadblow so thought I’d better come here first.

Essentially I’m getting 2 different values for the spacing of planes depending on the order I select them. I know they are within 0.3mm(physically checked, both using the DRO and also just locking the Z and moving over to check) and using single points it agrees with this observation.

However when I select say, plane 1 before plane 2, it’ll read 0.5mm(reasonable), and if I select plane 2 before plane 1 it’ll show 12mm. Any ideas?