Do not know about the names being interchangeable, but if during any census an enumerator puts down a name incorrectly that can often be the cause of some of these confusions. Also bad handwriting and transcribers mis-reading, I know that they are told to type what they see and not what they know the correct spelling/name should be.
Then there is the baptism/birth registration conundrum. I have in my family a child given one name when baptised 2 days after birth, but by the time he was registered a few days later was known by a different first name and baptismal name was used as middle name.
I also have an Ellen in my family, she was shown as Ellen on all census returns and even married as Ellen, when I finally found her birth certificate she was actually named Eleanor. And I only found this out from her death cert.
So names can be changed for all sorts of reasons, and our ancestors just seemed to do this to annoy us
