It happens to be the way the parts ordering computer screens are set up. The parts guys have to make a choice in that screen before they can get to the screen with parts numbers. The starters are probably the same for 2bbl and 4bbl.
My pickup happens to be a GMC 3/4 ton 4WD with a 350 4bbl. Most of the common SBC engine parts are the same for any GM/Chevy 350 pickup of my vintage, but I still make sure they pick the right model in the parts catalog, just in case the parts are slightly different.
If you want an easier way to check on the variations in parts numbers, try the rockauto.com online catalog. It provides information on different brand choices for the parts, and seems to have better pictures and specs than some of the other sites. You can also do a reverse lookup to look up a parts number and find out what other car that part is used on.