It's the same with my students' names. It really seems weird to me to shorten Christopher to Chris or to Angela to Angie, which is why when kids come up with new nicknames for their classmates, I can never get on board.
It was a losing battle to convince any of them of the actual name.
Besides the name shortening or lengthening or tweaking, there's also the substituting of an entirely different one. When encountering a classmate's mother, father, or sibling whose name is unknown, third graders tend to simply generate their own based on what they do know. Robert frequently asks to set up sleepovers with Donardo at dismissal. "Hi Donardo's Mom" he'll begin.
On Friday, I got to see an extension of this naming philosophy. Robert and I were walking in the hall, when we ran into Christopher's younger brother. "Hi Christopher Two!" Robert called out.
Do your or your students have any unusual nicknames?