MK3S Purge Location Problem

Hello hope some one can help as ive got a strange problem and cant figure out how to fix it.

using the purge location gcode I can get the nozzle location to line up perfectly at a z height of 60 for example, but when I then re-run the purge location test at a different z height say 20 this time the nozzle will not be lined up any more, then if i change the height again to say 120 z height then it will also not be over enough and not lined up, but then if i change z height back to the first height of 60 it will be perfect again.

its really annoying as it works perfect for very short prints where there is not a lot of z change but as soon as the print gets to high on the z then the purge location is off and wont work properly anymore.

Please if some one can give me some ideas of what I can change or check to fix this issue as its driving me nuts lol

Thanks