Never say never. They do, as otehrs have opined, sometimes sub planes. GRU-FOR, for instance, is a one-class route, and seats are always sold as such, but a few months ago we flew that route and there were two cabins-- though just one class of fare. We managed to score one of the bigger seats. Flight attendant said the plane had come from EZE. Just a one-time fluke.

TAM doesn't normally assign seats in advance, at least on domestic routes, as far as I know, so even in your case it doesn't appear they sold seats that don't exist, but did, at the check-in counter, assign seats that didn't exist (maybe because of a plane sub). People do switch seats when buying upgrades to exit rows, so maybe the computer system got onfused. All in all, I don't see anything in your post as proof that TAM sells seats that don't exist. Overbooking, perhaps, and lots of airlines in many parts of the world do that. Now, about the dimunition in size of food given on domestic flights (used to be meals then foil-wrapped sandwiches, now sometimes just cookies and crackers).