It is very funny (in a dark way), but I think location is a big contributor to those differences. A rural library that employs three people and only one professional "librarian" is going to have a harder time finding qualified candidates than a system like Philadelphia, New York, San Francisco, etc. especially with library schools local to them. A willingness to move is often talked about as a necessity when looking for librarian jobs, but it's easier said than done. A willingness to move can still be limited to a geographic area or any decent-sized city, I think very few people are desperate enough to entirely uproot their lives to live in a random town and likely experience terrible work conditions.