How come during Boethia's quest as well as Peryite's quest when the player is supposedly transported to their realms of Oblivion do both realms look exactly like Mehrunes Dagon's Deadlands?

Also... are we SURE that this is the Deadlands as the main realms invading Tamriel? Because it doesn't fit the description of the Deadlands in books such as The Doors of Oblivion.

Does it ever specifically say that it is the Deadlands invading and not just some generic realm?

Both questions answered would be greatly appreciated as this has been bugging me for some time

The game used only 7 versions of the Deadlands that would randomly be chosen in the non-scripted gates. There probably wasn't enough time to make a separate plane for Peryite during Oblivion's development.

Yes, I think they got lazy with the Boethia and Peryite realms. When they got around to doing SI, they made Sheogorath's realm different.

The lore doesn't seem to have Peryite's Pits being all that different from the Deadlands, though. It's still a low, fiery part of Oblivion.

I'm not sure that it's Peryite's Pits in that quest. The dialog says "I will transport you to the plane of Oblivion in which they are trapped." implying that it's a random piece of nowhere, and they didn't actually get where they wanted.

Boethia's is hard to justify, but the lava does contain the action well, without blocking the view.

For no less reason than to flirt with the Morrowind fans, I reckon

It's madness to think so, and madness to think otherwise. Madness, I say!

They do look a bit similar, don't they? Giant mushrooms and all. Magic mushrooms.

As said above most likely time constraints. When they had time to do it like in SI they made the realm amazing.