The airport it's surely correct. However, the real life KLAS airport has about 100 ft altitude difference along its area. But, since Flight sim doesn't really support sloped airports (not without causing lots of side effects with AI, ground lines, etc.) we don't have any other choice than flattening the airport to the published altitude.
The default mesh from Microsoft was processed to smooth out elevation differences taking into account that all airports would be flat, so you don't see any issues with the default mesh, but if you install 3rd party meshes, there aren't usually processed to do this, so the problem will show.
Since you use FS Global, they have a companion product called AFM (Aerodrome Flattening Meshes) which does the same kind of processing to fix sunken or elevated airports:
http://www.fsim.net/AspNet/Pages/afm.aspxand it's a free DVD (just shipping) for FS Global registered users, and it's included in the FS Global Ultimate edition.