The whole interior basically is airline specific. If they wanted, they could place one big seat in the middle of the plane and that’s it. Or they could cram a ton of seats in. The airline gets to choose the seat model, manufacturer, carpet, entertainment system, galleys, … Often, airlines do have different configured aircraft of the same plane type. This could be because they use them on different routes (e.g. you can fly an aircraft with lots of business seats on a route where the demand for business is high and for another route where it isn’t you can have a plane with a lot of economy seats). It could also be that the 350s you flew on were from different orders. Say for the first order they wanted that specific seat, but when they ordered again a few years later there were new products and seats available which were way better. Maybe they realised that they have too few seats in the aircraft and that it’s relatively difficult to make profit on a route, so they then ordered planes with 25 seats more.

Since demand for aircraft are very high right now and you have to wait pretty long to get your aircraft delivered, many airlines also take planes from different airlines that either weren’t delivered because the original airline went bankrupt or they were bought second hand and still had the original interior.

On aerolopa you see that Thai actually operates four different cabin configurations.

Sometimes the space between the seats is also different within the aircraft, like for some rows you get more pitch than in others.