Names can easily get truncated and, when first working on a family, it can be difficult to know whether a name has been truncated or not.
A family who went by Noon occasionally had their name spelled “Noone.” That was easy to discover as “extra” es on the end of a name are not too uncommon. The family’s name was might have actually been Noonan–it gets that way in a few records as well.
Or someone could just have thought their last name was (or should have been) Noonan. Sometimes variants are created because a clerk or enumerator thinks that’s what the name should be.
If a name is relatively short, always consider the possibility that the name you have was actually part of a longer name.