Yeah, that's how it should be. But if you were to go into the Morrowind engine and use the console with PlaceAtPC and then started combat with Ur Vs. Vivec and Almalexia , the two Tribunal would certainly win. They're stats were set higher than Dagoths. I am guessing that the devs assumed you would be facing Ur at a lower level than Almalexia.
http://tvtropes.org/pmwiki/pmwiki.php/Main/GameplayAndStorySegregation Essentially, the gameplay of games doesn't always accurately reflect the story, the power levels characters have in gameplay not accurately reflecting what they have in the story is one possible case. This is probably usually for balance, especially if it's in the case of a player character being stated to be so powerful that the enemies in the game wouldn't be any challenge at all, or because some bosses, if they were as powerful as the story says, would be so difficult as to make the game not fun. In this case, it's probably because Dagoth Ur is the final boss, so he needs to be weak enough for the player to be able to defeat him (though I'd say that once you've actually made him killable, he was weaker than he needs to be.) Whereas Vivec was not a character you had to kill, so he could be made stronger. In the case of Almalexia, since she was the final boss of an expansion aimed at higher level characters than the original main quest, gameplay demands that she be stronger than Dagoth Ur. Bethesda couldn't have just written the Tribunal to actually
be stronger than Dagoth Ur in the plot, as that would undermine the purpose of the main quest. If the Tribunal could handle Dagoth Ur, Morrowind wouldn't need a prophesied hero to kill him.