why is one double-hung parting stop shorter than the other?

century old doublehungs. I know there's a reason. And I found out when I did my first resto and both stops were the same length. Maybe my brain is too addled. But I don't remember why I ended up taking a saw to that bottom inch. Esp. since I later removed it completely anyway. So why install it short of the sill? Back then I had a good reason, because when I rebuilt the window I purposely kept one stop shorter. And it seems to have something to do with the beveled sash meet.
Now...

why is one double-hung parting stop shorter than the other?