It appears that none of us actually own this particular mould. However, if I needed a .310" round ball mould, I would not hesitate to get it. As several of us have pointed out, different lead alloys will drop out of the same mould in sightly different diameters. Mould manufacturers who make round ball mould blocks typically intend them for pure lead, and they sometimes say that in their literature. Maybe the seller was trying to allow for this phenomenon in his wording.
Only the writer of that description can tell us why he (or she...) used the word "nominally." It appears the mould shown in post #1 was from Buffalo Arms. All of the Pedersoli round ball mould blocks from Buffalo Arms (and there are a lot of them) are described as "nominally" some specified diameter. On the other hand, Dixie Gun Works has some of the same Pedersoli mould blocks, and they don't use the word "nominally" at all. They just list the size.
If you really need to know, maybe a call to Buffalo Arms might yield an answer to your question. This forum is a discussion group. If you throw out a question, it will be discussed. That's what we do. Many times some interesting and useful information you had not expected comes out of the conversation, and that's why we keep coming back. However, if you feel your question has not been answered, maybe the seller of that mould would be the one to ask.
Good luck.
Notchy Bob
