Arcade OSD lets me scale any resolution...except 640x480?

[vrada501]

For a while I thought I had a handle on understanding how Arcade OSD works, how to use it.
With it I've been able to scale the resolutions modelines gives me, the super resolutions and the resolutions lower than 640x480 that work, by messing with the back and front porches.

The only resolution I'm having problems doing that with is 640x480 (Interlaced). Any difference I make to the portions makes zero difference, the scaling stays exactly the same.
And the range of numbers these porches can be in are in the hundreds which seems weird to me. What's going on?

I'm getting modelines from NTSC TV preset. And just in case it matters, on a Sony Wega.

[Calamity]

You can't modify the current desktop video mode.