I guess it's where water and moisture collect the most and the chrome is not always the best in the gusset or doesn't quite cover it - bb and and behind seat post are other common places ?
it always lifts the chrome and it the hardest to get clean - most of the time all you can do is remove the rust in a OA bath and then hand polish up - it will always show the black bear metal up - but you can try to blend it in
was the same on my other one
