it's because of slang

according to JC, you are well-hearsed when you're knowledgeable about something or just really well-prepared.
me: you can say "well-rehearsed"
JC: yea, it comes from "well-rehearsed", but because it's "slang" you don't need the "re" so it's just "well-hearsed."
the rest of us: uh...i think it's well-versed, to be well-versed in something
jessica: oh well whatever.