it exists at 290-300 to encourage people to raid. At 290 you can do the raid just fine, so it becomes more optimal to play the harder content to make that last gap - which is pretty standard game design.
I think he said it doesn't slow down after 300. Just that 290-300 jump.
http://www.neogaf.com/forum/showpost.php?p=180283992&postcount=12947
he also mentions in this post that he was going to look into raid gear being able to drop at 300
edit:
http://www.neogaf.com/forum/showpost.php?p=180280492&postcount=12831
IMO the reason 310 exists is due to the new light level math. 310 is to give you a 'crumple zone' of sorts to hit the max content level of 300, so high rolls can pull you up over it even if you have sub-300 items on you still.