Different Filter Pressures Depending on Valve Position?

I have one main drain and one skimmer. As a matter of habit, I leave the main drain closed and use only the skimmer. Pressure has held at a steady 11.5 since installing new cartridges a few months ago. In anticipation of going on a trip for a few days, I went ahead and opened the main drain so there would be an additional source of water if the skimmer were to get blocked. Anyway, I noticed this AM that the pressure was running at about 15. I returned the valve to the "skimmer only" position and it went back to 11.5. Selecting "MX Drain only" resulted in about the same reading. There is no cavitation, flow seems normal and there are no bubbles in the pump housing under any configuration. Is the higher pressure an expected result? I would think (which I know is dangerous!) that if anything the pressure would be lower?