When might it be reasonable that a question is closed as off-topic because it "does not appear to be research-level mathematics" while one of many narrow special cases of that question, posted separately, gets 7 up-votes and its only posted answer gets 13?
I can't provide a characterization of such situations, but it is not hard to come up with new ones. For example, a question like "What can you say about algebraic curves with nodes?" would be rightly closed, while a well-written question about the existence of a plane curve of a certain degree and a certain number of nodes may be well-received.
In general, it helps if the question appears to have had a substantial amount of focused mathematical thought behind it.
Edit: In response to comments, let me refine my example. If someone asked the question "What do nodal algebraic curves generally look like?" together with the text "I sketched the graph of $y^2 = x^3 + x^2$ and was startled to find it looking like a fish." it would be rightly closed.
Regarding the specific situation, it looks like Neil Strickland's comment answered your question (inasmuch as it could be answered) in two sentences. Your later question was substantially better than the first one. In particular, because you were seeking a specific characterization, it was an actual mathematical question, not just a special case of the first question.
If someone is calling you names on MathOverflow, we moderators would be interested in hearing about it. On the other hand, if there was a heated email exchange out of the public eye, I don't think it is reasonable to bring it up here.