This is due to how the VMF format works when compared to the RMF format.
VMF looks something like this (sorry if this is a bit wrong, guessing from memory here):
solid {
face {
"plane" "(0 0 0) (0 0 512) (0 512 512)"
"material" "tools/trigger"
// Blah blah more stuff here
}
face {
// And so on
}
}
The important thing with this format is that the brush vertices are not actually stored in the VMF format, just the face planes. When Hammer loads a VMF, it performs a plane intersection algorithm to obtain the values for the vertices. Because it uses floating point calculations, you get the small rounding errors you see in your screenshot.
The RMF format, on the other hand, is a serialised binary C++ data structure, the very same structure that is used in the Hammer code for display and calculations. Roughly, it's something like this:
class CMapSolid {
CMapPlane plane;
CMapVertex vertices[];
CMapMaterial material;
// And so on...
}
This structure of course has the actual rounded values for the vertices and these are used when the map is loaded. You still get rounding errors when the vertices are off-grid and when converting to the MAP format (which is what HLFix partially mitigates), but if your vertices are on the grid you won't ever get any rounding issues in Hammer 3.
As a bonus, let's look at the MAP format (too lazy to look it up but it's something like this):
(0 0 0) (0 0 512) (0 512 512) "AAATRIGGER" [0 1 0 0] [0 0 1 0]
The MAP format only stores the planes, just like the VMF format. The interesting thing about this is the MAP format is what the Goldsource compiler actually uses to compile the map, which means that the advantages of the RMF format are lost when compiled. This is still better than Hammer 4, where the precision of the vertices are lost in your VMF file, rather than just in the compiled BSP. If you saved your Goldsource maps in the MAP format instead of the RMF format, Hammer 3 would have to do the intersecting planes algorithm to calculate the brush vertices, resulting in the same off-grid issues that happen when using the VMF format.
