When it changes, there is sometimes a note in the notice of coding telling you why. If you don't understand it, then phone and ask. My code went down for this year and they said it was because I was receiving a benefit, which I wasn't!!
P60 shows last years code. The allowance went up this year to 9440 so you get more money tax free now. Most PAYE workers without any complicated circumstances have changed from 810L to 944L this year.