ubuntu å vs osx å?

ubuntu å vs osx å?

Not sure if this is a ubuntu or osx question, but I'll start here. I'll
leave it to the mods to move the question to AskDifferent if more
apropriate.
I moved a file from ubuntu to osx using scp on the apple machine. I edited
the file on the apple machine. Then I moved the file back, again using scp
on the apple machine.
The filename of the source file was Documents/trettiårsfirarätare.
The filename I got back had the name
Documents/trettiårsfirarätare.
While these might look similar, the letters å and ä is actually different
between them. At no point did I change the name of the file.
This makes little technical difference to me, I just changed the name of
the file back to what ubuntu considers å and ä, but it tickled my
curiosity.
Can you explain to me why this happened?