Basically, when a mage goes invisible, they seem to briefly enter another realm or reality. So if a mage stood still, cast invisibility, and continued staying still, not only would other people be unable to see them anymore, but could then stand on the very same spot the mage was standing on and not come into physical contact with them.
This would contrast with rogues, who do everything that the mage does, sans entering another realm. Note how Saurfang was able to pluck one of the rogues from the air while Thrall threw dirt at another. They would not be able to do that if the rogues had truly gone invisible.
So stealth, as it is depicted in game, is just a lesser form of invisibility where the rogue makes it difficult, but not impossible to be seen.