Why does OS X use swap memory when memory pressure remains low?

My system (Yosemite / 10.10) has been paging out to swap memory despite my memory pressure reading less than 50% over the last 7 days (tracked with iStat Menus). How is this possible? It would be one thing if memory pressure exceeded 100% at some point in the last week and then the swap file was activated, but that doesn't seem to have happened at all and yet I've got 1.25 GB of 2.00 GB being swapped right now, with 36% memory pressure. Shouldn't the new memory model introduced in Mavericks behave differently?