What the...?

Okay, explain this to me. At my college, a parking permit for the full year costs $170 (incidently, I believe they quite jack up the price every year), which is not too terrible for a year. However, the single semester permit costs $135! That's just not right. Why are they picking on those of us who, for one reason or another, only need a permit for the one semester? Seems like a rather mean screw-over to me.