Change to world reverses direction of volume light?

[Johnny]

I am using a volumetric light in a scene to simulate smoke/fog. I had to change the texture to World so that it would look right from the cam perspective, but when I rendered clips with "World" checked in the texture, the motion OF the texture was backwards!

The fix was easy enough...but why would clicking "World" make a texture go backwards?

I guess I'm asking because the answer might help save me from this mistake again.

thank you!

J