Just because your light level is over 300 does not mean you will get 310 exotics. As long as your light level is 300 or better, you have the chance to get 310 exotics. ITS A CHANCE NOT GUARANTEE. All you get is the chance of exotic dropping at 310 but its not a huge percentile, I assume its around 10-30%. When Xur sold the exotic engrams I been over light level 300 and bought 5 engrams on each character. When I turned them at least 1 of the 5 were 310 so I assume it has a 20% chance (1/5).