Had this same problem. I scratched my head for at least an hour taking the bumper on and off trying to figure it out. Eventually one of the guys here said it felt like something was pushing the bumper down as we could push it in a bit and it would kinda "pop" back out. We tracked it down to the front lower air dam(hard plastic piece under the bumper/radiator) and the bottom of the bumper lip/tabs, we messed with it for a bit until everything lined up perfectly, and voila no more gaps!
Please note we are using the APR Carbon front lip, so we thought that may have been contributing to the problem with getting everything lined up as we had no flex in the bottom of the bumper, but hopefully this may help some of you, or at least give you an idea on where to look.
Thanks!
Rick
Edit, I stole this picture from @
DC-FRS, hope he does not mind
The large plastic air dam(#22 in pic) was not sitting right with the lower portion of the bumper and was sort of forcing the bumper down a bit. I had to push, pull, prod, poke, talk nice to, and yell at it until it was all lined up perfectly.