[Ghoulish Delight]

So there's your answer. Quirk of Windows file system networking. If it's mapped as a drive letter, it looks at the drive info, rather than the directory info.

That's partly because the "remaining capacity" measure doesn't make much sense in a per-directory measurement. Notice that when you go straight to the directory it only gives you the size of the directory, not any sort of "remaining size" measure. So from the standpoint that the mapped drive is a drive, there's no other way to present it.