Frequently in my Ancestry UK hints and searches, I find trees which match my own except that many of the people on them, from the 17th and 18th centuries, have middle names. In every case so far, there has been no documentary evidence to back up these additions.
Why do people do this?
I've found the same thing and it's always mystified me as well. And it's rarely because they've randomly picked a record of the wrong person who really does have a middle name, and they've started barking down the wrong tree. Because the dates and places of documented life events for the right (middle name less) person are correct.
I have several people in my tree have genuinely acquired a middle name - meaning they added it themselves.
A brother and sister who after they were orphaned added their parents names (the boy took the father's and the girl took their mother's, of course)
My great aunt who for some unknown reason added Kathleen when she married (not a family name).
A 2 x great grandfather who added his mother's maiden name. Quite a common thing, but he was not baptised with it.
And another thing I've seen is "lengthening" names which the original might be considered a nickname. Probably to sound posher
4 x great grandmother, baptised and married as Betty, became Elizabeth on 1841 census (but form might have been filled in by her adult son who was living with her even though she was listed as Head of household)
Great uncle baptised and registered as Tom. But at death and burial he became Thomas. His death was registered by a coroner, but he was resident at his parents house on his burial record, so I would have expected them to have him buried with the name they registered him as.
I also have people who have "lost" a middle name. Maybe some didn't like their middle name and dropped it. But maybe some people never knew they had one. Before civil registration, the only document which would give the full name was the baptism record - and how many people would have seen their own baptism record? And even after 1837, how many Victorians would bother to look at their birth certificates, to check what their full name really was? Did they even need their birth certificate for anything in those days?
More recently. A friend of mine (born 1954) was always called J..... and thought her forenames were J..... E........ She married as J.... E....... and it wasn't until a couple of years later when applying for her first passport she needed her birth cert (which she had never seen and her parents still had) she discovered she was actually registered as E........ J......