Lidar Cloud to DTM Grid has "ghost" surface

jover_usgs

I have a lidar pt cloud that i have been editing to remove a building from being classified as "ground" - i did, its now unclassified, but when i go to make a grid out of just the ground points, it still creates a grid of the side of the building, where there are not visible ground points.

This is the building, unclassified is gray, brown is ground

i got to grid it using just the ground points....

and i get this surface where there shouldn't be one...

Any thoughts? Help? Thank you!