Originally Posted by
paperwastage
TOo many factors go into airline pricing...
Eg, 7pm night flight allows someone to work most of NYC day, and then arrive London to work the next day (plus skipping a hotel night).
If you want to know why the pricing is different under the covers, it could be a combination of different fare buckets available (the cheap bucket available for daytime but not nighttime) or fare rules (XYZ fare rules valid only for UA123 daytime flight). Both available if you use United expert mode
I think 7pm the OP has referred to is the fare pricing time, not the flight time.
Agreed, however, on the rationale, the dynamic in fares is governed by fare rules and availability of cheapest fare at the time of fare quote.