Four years late but I'd like to answer that this likely happened because they were two different body types. Barring any custom downloaded meshes/skins that used their own special body type (for instance, there were some "XXL" custom body types (often more flattering and/or realistic ones) that gained traction in Sims 1 creator circles that had custom body type codes), the Maxis body types for adults in the original game are skinny (skn, incidentally also the file ending for one of the skin file types, but this would be part of the file NAME), fit (fit), and fat (fat). (Children have one Maxis body type: chd.) In CAS, you have all of them to choose from, but once you save the family, the body type is locked in and unless you're using certain objects or external programs that let you set to a different outfit that's a different body type, the options you'll see are only from the body type the Sim is at that exact point you looked.
It's very likely that the Sim with less options was in a fat or skinny outfit (so only able to see FAfat or FAskn skins) and the Sim with more had a fit outfit on (only able to see FAfit). Fit tends to have WAY more options (both vanilla AND in terms of what you're likely to come across with downloaded skins.
