Yet another issue I was faced with too. Before I shot my first SG I consulted with an experienced shooter regarding this same barrel labeling / misunderstanding.
I had just taken the Hunt Ed Course and picked up my first SG. I was eager to shoot it at the LIVE FIRE the following week and after receiving instructions on what SHOT SIZE to buy I went the store and noticed that all 9, 8, and 7 1/2 sizes only came in 2 3/4" sizes... Ugh I thought. My barrel said USE 3" SIZE. I went ahead and bought the 2 3/4" size shot due to the fact that in my search for my first SG I had read many many gun specs and all had labeled their SGs as 2 3/4" AND 3".
Before I took my first shot I consulted w/ the PRO at the range who reassured me that as a general rule the label is for MAX SHELL SIZE and that 2 3/4" would be fine.
My manual states that the barrel label should read 2 3/4" AND 3" SIZE for my FIELD model. This would be to distinguish it from the SPORT model of the same name that takes ONLY 2 3/4" SIZE.
Something tells me this type of labeling is probably introducing an industry standard listing MAX SIZE on certain makes...