I discovered an odd, yet possibly useful(?), effect of %sp.
I had two %sp commands, with the same data, for one window. The effect is that the first %sp sets the window absolute position. However, the second %sp offset the window position by exactly that amount from this new starting position. Stated another way, if the coordinates (screen absolute) were (100,200), the final window position would be (200,400).
I can see using this 'feature' to advantage when a set of windows might overlay each other (like when successively opening folders in windows). That said, I have not tested that hypothesis. I'm just glad I found the extra %sp. Was driving me even crazier!!