Half dollars were unfortunately the target of several "get rich quick" schemes in the past few years. Strictly in terms of mintage figures, the 40% silver halves should be a good 30% of the total circulating population--about as common as the 1971-1977 halves. I've seen plenty of books that tell tall tales of "free silver" to be had by searching halves. This draws in plenty of people who are blinded by the idea of an early retirement and have no respect whatsoever for the hobby--they will literally deface anything that isn't silver in the false belief that they are making their job easier. I've found plenty of beautiful BU halves with an ugly black or red X right through Kennedy's face--others on here have found marked error coins, proofs, and other collector only coins that have been defaced by these silver bugs.
It's an ugly truth you have to face when searching half dollars--I guarantee that the person who triggered the "no more than $50" rule at your bank turned in a few marked coins himself.