A much more mathematically inclined friend of mine showed me the full breakdown, but it's actually slightly more than that because Critical hits and Nat 1s come into play to skew the averages a bit more.
Additionally, the rules of 5e literally make Advantage/Disadvantage equal to a +-5, at least in terms of calculating Passive skill checks. PHB page 175, "If the character has advantage on the check, add 5. For disadvantage, subtract 5"
Right, speaking strictly about attack rolls there is critical hit/miss. The flat equivalent depends on AC but its somewhere between +-4 and +-5. That flat modification from PHB is too generous for my tasting, especially when ability and skill check rolls don't use critical hit/miss mechanic. But whatever.
Advantage or disadvantage has also another builtin "feature", they don't stack, while normal flat modifiers usully do. They also cancel each other. All of those are reasons why I think high ground rule should not use advantage/disadvantage rule but something else. (EDIT: Adv/Disadv. should be quite rare, undeniable conditions)
