Apple is going to setup more premier feature for "+" version of iPhone in the feature, and I will not be surprised 7+ get QHD, but 7 only get 1080p. Both still are improvement from 6s/6s+.
The reason is profit margin, by giving more feature but able to charge $100 more, Apple is able to increase overall $$$ revenue. Parts different is not going to be anywhere close to $100, but they get $100 extra for customer choice between regular vs plus.
Another reasons is there is simply more room in "+" than regular, hence easier to design better product, included better display, and allow Apple to use slightly larger size component if needed, without sacrifice battery.