Like other game mechanics, a player spends Monarch Points to increase stability, in this case Administrative points. Since these are also the points that go towards increasing the number of Ideas and revenue buildings, it is less likely that a country will always be at peak stability since a player may plan on investing Monarch Points in other things.
The base price for increasing stability is 100 administrative power, but overextension and lack of religious unity can increase this cost significantly. Some ideas and advisors decrease this. There are also events and other random factors also modifying the price temporarily.