Up-lining by at least 2 line weights for shooting heads has been the norm for decades. The reason is that a standard 30-foot shooting head usually has no significant or contributing weight in the shooting/running line. With a standard line - WF full length - the overhang, 3 to 10 feet or more adds considerable weight so it's best to stick with the AFTMA line weight if you want a delicate presentation. That said, I always up-line by at least one weight for a quicker load at distances under 30 feet. The average amount of line extended while false casting a full line is more than 45 feet at times. More weight...
You can't do that with a shooting head so you need the extra weight to get a proper load with only a few feet of overhang. Ditto for the RIO Outbound Short lines which were designed to act and perform like regular shooting heads only with an extruded running line.
I hope this makes sense to you. There is an article somewhere in the archives written by Scientific Anglers, Bruce Richards which explains this better than me. I'll look for the link. You can also search the site by using the custom Google search bar - type in key words like shooting heat or shooting taper.