Ground Balance Question

[Blink]

I'm not new to metal detecting, I've had many different machines over the years. But I'm a little confused as to why my ground balance on my AT Pro is working the way it is.
When I first turn my machine on I set it to pro mode, zero mode and look for a piece of clean ground to ground balance on. Why is it if I auto GB and get say 75 and then I move to another clean spot and I get 80, then I move back to my original GB spot, shouldn't I get the 75 I started out with? The GB will be 80 at the original spot but it doesn't move back to the original 75. Is this normal ? I've never been concerned about it before but I'm wondering if there may be something wrong with my GB. I tried the same thing on another machine and it worked the way I thought it should. Maybe I'm doing something wrong !
Any help or suggestions would be greatly appreciated.

 

[John LA]

GB on my ATPro works a bit different than my other machines. If my auto GB is 90, I manual set it to 70. If my auto GB is between 50 and 70, I manual set it 10 to 15 below the auto setting. At the beach (salt water) I manual set the GB 4 to 6 below the auto setting. I get better depth with the manual setting. A person at Garrrett to me my ground to mild or neutral?