Changing one of the three "dimensions" changes the other two for a given geometry. If the STP increases, so does the effective length, the cartridge offset angle shallows.
This only makes sense...
As P-to-S increases, effective length must increase (logically). Also, as P-to-S increases, overhang decreases and cartridge offset angle shallows.
Taken to the extreme (as P-to-S goes to infinity), the overhang goes to 0 (as P-to-S and effective length becomes equal) and offset angle goes to 0 as well. This is what a linear tracking arm attempts to mimic.
No thanks necessary for me pointing out the obvious. I'm here to help.