Ok so engrams don't scale up, they are fixed. Blue items continue to go up based on your power level, but once you hit around 265, they will always be ~5 points lower than your overall power level.
edit: oops meant to edit my older post but made a new post instead.
That I've seen they drop exactly 8 lower, no more no less. Sometimes because of the exotic bug *(where the game only counts one exotic armor and one weapon in it's calculation) or something else they will drop 7 lower (meaning the game decides your power is 1 higher than it is, possibly because of something I'm not accounting for). Legendaries go by the same rules.
You may have gotten confused because you weren't always running your max gear.