Once you reach level 300, engrams have a chance to drop at either 290 or 310. That's how it works. It's all chance, with a little bit of BS sprinkled on top (cause let's be honest, you're gonna get a duplicate on average). Supposedly, the 290's are supposed to have better rolls, but I've been lucky in getting 310's the last few times, and they're gear I wouldn't use anyway, to know for sure.