SPOFS:
Units are always screen pixels
Positive numbers move the object down/right
SPHOME:
Units depend on the object scale
Positive numbers move the object up/left
BGHOME:
Units are always screen pixels
Positive numbers move the object down/right
BGOFS:
Units depend on the object scale
Positive numbers move the object up/left
My theory is that BGHOME and BGOFS were switched by accident, and by the time anyone noticed, it was too late to fix without breaking every game that used BG.
This seems plausible to me, and the detail about the units they use is strong evidence for your theory. If not for that, however, I don't think I'd be quite as convinced, because I think BGOFS using positive numbers for up/left might be more intuitive anyway.
If you start a sprite at the top-left of the screen and increase SPOFS's coordinates, the result is that the sprite appears to move down and right.
Similarly, if you place a stationary sprite in front of a background, and increase BGOFS's coordinates, the result is that the sprite appears to move down and right, in relation to the background.
Even though it's actually the background that's moving up and left, people tend to interpret the background as a stationary thing that the camera is panning across. In other words, you could think of BGOFS's coordinates as being the coordinates of the camera rather than the background itself.
Consider this as well: you're almost never in a situation where you want to move the camera outside the boundaries of the background. BGOFS using positive numbers for moving up/left is well-suited for this, as it means you'll be working exclusively with positive numbers. Conversely, BGOFS being down/right would mean exclusively using negative numbers.
Quite honestly, the fact that the sprite and BG commands use the same naming convention but behave pretty differently (almost backwards) makes this more unintuitive than anything else.
