Different prints with projects built on different computers, same grabcad version - why?

I noticing odd extra printed lines or ribs when GrabCad projects are created on a users computer vs my computer attached to a Stratasys J750 printer (both using GrabCad 1.20.50.42688).

In one example, the top flat surface has a small extra raised area in a particular shape, which persists even if I rotate the print direction 90 degrees. In another example, I have a flat top surface with a set of ribs underneath it in a particular pattern. When I use the project built on his computer and emailed to me, or he sends directly to the printer, the ribs are getting mirrored (about 20% height) on the top of the top flat surface. Both his computer and mine are running the same version of GrabCad. We never had such issues until he starting sending me project files instead of STL files. Has any one else ever had this problem?
(Sorry, but I cannot send pics of confidential designs to explain this better)