Hi, Joe,
All questions are good questions. If you are wondering about something, odds are 10 other people in here are wondering about it, too.
I agree with the previous description of breakthrough seizures. And I'll add a bit. If someone's seizures have been well controlled and then they have a seizure, it's a breakthrough seizure. My neurologist counts all possible kinds of seizures, including auras, as breakthrough seizures. He wants them ALL under control. Some doctors don't count auras, some do.
In addition to changes in meds, diet, stress, routine, sometimes we do things that affect our absorbtion of the anti-seizure medication. That was true for me once when I had an upset tummy and took Pepto Bismol. I thought it only affected your stomach and wouldn't affect absorbtion in the intestine, but I think I was wrong. I had a bunch of breakthrough seizures later. (Or, it may have happened because I was sick - who knows?) Also there are some meds where you can't eat grapefruit. I have a suspicion that excessive drinking or meds that are also metabolized through the liver (or some meds, the kidney) also delay or prevent the anti-seizure meds being metabolized.
Some people get acclimatized to their anti-seizure meds and they quit working as well, and they get breakthrough seizures, too.
All this are reasons why a seizure journal is so important. It helps track all this stuff, which helps us figure out why a breakthrough seizure happened, and how long it's been since we've had our last seizure.