GMAT Sentence Correction (SC) | Re: A number of good reasons and A wealth of good reasons.

Why is "A Number" considered Plural, but "A wealth" considered singular?
I did not receive a concrete reply as to the crux of the problem. I did not consider the two phrases different (as they convey the same meaning ).
Is it because "A number" represents a part of a whole, while "a wealth" doesn't have a super-set?
If so, then is it a general rule to use a singular verb form for a collection without a super set and plural for a word which represents a
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