That GPS signals aren't as good in a vertical plane as in a horizontal plane.  Roughly a 2:1 ratio.

So, if your system varies 2"-4" horizontally, that would mean it could vary 4"-8" vertically. Basically, only RTK is going to be consistently accurate enough for tiling.(as Chad stated)

I am just passing along the info, I viewed, but it did make sense.

[Added Note: They (Hefty Bros.) run Gold Digger plow with Intellislope]