How do a{n}? and a{n} differ?

Nothing. The article states:

The {n} quantifier matches the preceding element exactly n times, where n is any integer. {n} is a greedy quantifier whose lazy equivalent is {n}?.

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The {n}? quantifier matches the preceding element exactly n times, where n is any integer. It is the lazy counterpart of the greedy quantifier {n}+.

Notice the text is exactly the same. Basically, adding ? does not change the behavior of the quantifier. It appears that .NET's regular expression engine supports {n}? as a alternative to {n}.

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Interestingly, this article does appear to contain an error:

The {n,} quantifier matches the preceding element at least n times, where n is any integer. {n,} is a greedy quantifier whose lazy equivalent is {n}?.

This is wrong. The lazy equivalent of {n,} is {n,}? which is not the same as {n}?.