As Umberto told me already that this is not a problem of the scenery cause it has got the right elevation but not the mesh-add-on "FS GLOBAL 2008 X" (for FS 2004 and FS X) I use.
Just to be more precise: the issue is not the mesh being "wrong" or not precise enough. Instead, is exactly the opposite: it's *because* the mesh is "right", that the scenery shows this effect.
The reason is, the real world KLAS (as many airports in the world) is not flat across all its area, there's a 100 ft elevation difference, for example, between the east/west side of Rwy 25.
But, since FS9 or FSX don't support sloped airports and runways, the airport has to be flatten to the elevation of the published QNH. This will create a visual difference in areas were the flattened airport meets with the mesh, which is of course similar to real world so it IS sloped.
The default meshes from MS had some pre-processing, that smooths the mesh in an area 1-2 km around every airport, so the visual effect do not get noticed, but this technique is not always used by mesh authors, that's why the effect is more visible with 3rd party meshes.
Of course, this issue is more relevant to FS9, were the use of 3rd party meshes is basically mandatory. Since FSX already use SRTM 90 m meshes by default (at least in the US and Europe), and this mesh HAS been pre-processed as explained above, the effect is far less obvious in FSX.