When my machine was new, it seemed the belts had a little breaking in (stretching) to do.
Z down low was inconsistent, like yours.
You only want the v wheels just snug enough, that it stops turning with finger pressure.
I also took off one of my springs. It had a hard time pulling both springs far enough to reach the spoil board.
After a few times, and z belt not being really loose, it should line out.
Temps also had slight effect. Starting off cold, it will stretch, just a little after a couple minutes.
After the "break in" it runs consistent, now.