Reasons for dramatic differences in pay for adjuncts?

I recently came across a survey of pay for adjunct faculty at California community colleges. It doesn't surprise me that rural schools pay less than urban ones, but there are some pretty amazing discrepancies between schools that are close together geographically. For example, the highest pay for a part-timer with a PhD is 50% lower in the North Orange County district (where I'm full-time faculty) than at Mt. San Antonio College. These two schools draw students from adjoining areas, and are 25 minutes apart on the freeway. Community colleges in California basically get their money from the state's general fund these days (not from property taxes, as in the past), so funding per student should be equal.

Are these wild contrasts present all over the world, or are they a specific to community colleges in California? Why do these huge differences exist? Are they evidence that this labor market is one that economists would describe as inefficient, with people not being able to make rational choices based on accurate information? I wonder if they simply reflect the political and fiscal attitudes of the schools' boards of trustees.

related: Why is it that adjunct faculty positions pay so little?