Oh, like it's the surface of the sun? Eeenteresting....
Also, to the OP: Read the http://www.imperial-library.info/content/fight-one-eating-birth-dagon*. It explains everything.
*See, it's funny because Dagon was Born This Way. Hurr hurr.
That's not very helpful. It's not readily apparent from the fights that this is the case. It's better if you make the argument or http://www.gamesas.com/index.php?/topic/1091564-noticed-something-about-destruction-magic-icons-in-oblivion/.
That said, I'm not entirely convinced it is the case.
And a few other similar arguments have been made as well with other actors:
http://www.gamesas.com/index.php?/topic/1183791-the-redguard-pantheon/
http://www.gamesas.com/index.php?/topic/1195761-ruptga-isnt-akatosh-hes-magnus/
http://www.gamesas.com/index.php?/topic/1182987-magnus-sithis-and-lorkhan/