Our local DM notes that an even ECL = CR match between a single PC and a single monster is intended to be a 50/50 break point. This varies fairly widely through the monster manuals, but run your one 6th-level PC up against every CR 6 in the book and they should get killed roughly half of the time.
But for the OP, dystmesis put it fairly correctly.
CR is a measure of how powerful the creature is as an opponent for your PCs (and isn't very good at doing so without factoring in things like terrain, PC abilities, etc). By this yardstick, abilities like True Seeing and Teleport don't actually mean anywhere near as much, since the monster will be dead in 6-24 seconds.
ECL is a measure of how powerful the race would be if used as a PC and took PC class levels. By this yardstick, constant True Seeing and use-at-will Teleport is going to ruin many, many plothooks and therefore is considered much, much more powerful. Thus, a creature with abiliites such as these receives a much larger adjustment to ECL than it does to CR.