Directly from Bungie's post:
"Available in Legend and Master difficulties.
Lost Sectors feature +20 combat delta (rather than -15 or -20)."
This being the only information stated by them on this subject, it makes sense to infer that they're say WE get +20. Since WE get a -15 or -20 in Legend/Master NF.
None of their math or logic has made sense since Lightfall.
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But like...why you legend lost sector buff you +20 when legend nightfall nerfs you by -20? Now I know people are getting confused by Bungie sometimes, but sometimes you gotta use common sense..."legend" is a difficulty mode, why would it ever buff you to higher than what the enemies are. It's not "rookie" mode...
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DIFFICULTY OPTIONS Hero, Legend, and Master difficulties have been reworked for greater challenge and to accommodate new modifiers. Adept difficulty has been removed. Hero activity level is now soft cap + 20. Legend is Power cap + 30, and Master is Power cap + 40. Each has a scaling combat delta that keeps you under level in the activity and provides a smoother pathway to Grandmaster. I mean, Update for light fall clearly states it's going to keep you under under level in the activity. By your understanding of the wording, Master difficulty would give u a +40 to light level... I think the key part you're missing is what they mean by "combat delta".