It comes from the old english tradition... from the the german tradition...
The technically correct simple past is ment, while the participle may be ment or mended. No one says ment anymore, so people just change it.
Consider the word "past"
it comes from "to pass" and once upon a time the pp and sp were "past". But now people say "passed" because it is more regular. However, the word "past" remains as a noun, and unfortunately no one makes the connection anymore.
This word, to mend, is similar.
Both "to pass" and "to mend" come from french, and route through middle english, which is why those archaic "rules" still apply.
Some other neat things the past part. of "to put" was once "putten". weird, huh?