This was something that bugged me early on, especially since in the Warrior storyline it seems like there's an actual body with Sadow's armor in the tomb. But since the Clone Wars gave Darth Bane a tomb there, I've just come to terms with the idea that any Sith Lord worth the name is gonna have sycophants who're gonna get his body to Korriban... for some reason.
I wouldn't give it much thought, even though I did notice it back then as well.
For some reason, Bioware just chose to reuse that armor over and over again, throughout the game... You can see that EXACT
armor in other locations like the Athiss flashpoint or even Lord Grathan's estate on Dromund Kaas.
In other words, one may assume that said line of clothing was fairly common back then or just maybe the dude interred there was a high-ranking member of the Sadow'een. Too many possibilities really.