Powerful engrams are not guaranteed upgrades. They should be, but here is the thing: Take your current inventory and make it highest light possible. It doesn't matter what it is, but let's say your 518 right now. Powerful engrams can drop at 518-523. Why also 518? Because your probably not 518 at some slot right now which is the major issue. Let's say you have a helm, arms, and all weapons at 521 respectively and you get a helmet. Your helmet slot can only be made better by a 522 or 523 drop. Yet you might also get a helmet at 518. That is what a powerful engram guarantee. It means your highest light, based on current inventory, cannot be made lower because of it. That's all.