Stats change when switching specs?

Hey all,
Might be a bit of a noob question, but why does my mastery stat go down when I switch from balance to resto? It goes from 24% as balance to 11% as resto though I didn’t remove any gear. My abilities also do about half as much healing in resto than in balance, which seems strange. Can I not switch between specs even though they’re both intellect based?

Different masteries scale better/worse but your total mastery is the same, some masteries scale worse due to how good they are and need the reduced scaling

Pesty is right. It’s one of the reasons mastery is so good on balance as a stat, it scales really well and has a high value.