Different pricing for adults vs kids? + lap infant question

I just booked flights for our family to Costa Rica. I almost never fly American, so I'm kind of confused about the price breakdown I got in my confirmation e-mail. $265 +tax each for my partner and I, and $199 +tax for each of our 3 kids, including the lap infant. First, I've never seen different ticket prices for kids vs adults, and second, I expected the lap infant's ticket to cost 10% of an adult ticket. It appears they gave him a seat (I went to choose seats to test it and it allowed me to select a seat for him). I thought maybe I made a mistake when booking, but I went back and verified that the only option for his age is "infant in seat under 2." I'm going to call AA in the morning to ask about this, but thought someone here might have some insight. Thanks!