I'm sorry, you are right. By one moment i forgot how the map channel parameter works here:
1) If map channel is 2, the size of the distribution map is taken from the UVW coordinates (in this case 1x1 m). In this mode, X/Y Units don't change the map's size, but only the density (basically it has the same effect than X/Y Pixels).
2) If map channel is 0, Forest generates its own coordinates, and X/Y Units modifies the size of the procedural map. In this case, 10x10 m.
So, setting dense.bmp as procedural Map (or embedded in VRayDistanceTex), Mapping Channel = 0, X/Y Pixels = 100, X/Y Units = 10m should generate the same result as using dense.bmp as Bitmap. But there is a small difference more: the map filtering changes the result slightly, so set map filtering to None.
Tip: to see visually how each parameter modifies the distribution map, use a checker bitmap for the tests. It's what i've done here.