Well, I understand that Bethesda probably like to publish games more than they like to release games, but explain me one thing:

The developer makes the game. Right?
Then why is the Bethesda engine reappearing in every published game? (I might be wrong. Correct me if I am.)

Just like FO3 to FO:NV. Beth made the first one, but Obsidian made the second one. And then again the same thing again with Dishonored.

Could someone explain this please?

Dishonored uses the Unreal Engine.

It is common for developers to allow other developers to use an engine for a fee. It doesn't necessarily imply they are associated in any way--lots of games use the unreal engine, for example, without being directly associated with the developer of that engine. Some engines are really flexible, so it is much cheaper to pay another developer to use their engine.

Dishonored does not use the same engine. New Vegas is game developed on the resources that Bethesda had already developed which dramatically reduced development time. Bethesda is for all intents and purposes done with the Gamebryo engine so we won't see any new games on that.

Bethesda's Engines:

Creation Engine

id Tech 5

While it is not mantatory to use these if dev'ing for Beth, it is worth while to do so as the licensing costs are going to be less.

What about creation engine? It turned out to be very unstable. Are they going to release an update of that engine, or an entirely new engine again?

EDIT: Thanks for the explanations!

When did they announce that? Skyrim was after all still made using a lot of Gamebryo's stuff, whatever they chose to actually name their engine.

Gamebryo was unstable too and they still released 3 games using it

And Skyrim's engine was so different from Gamebryo that I don't think it is really fair to equate the two.