Historical reasons probably. At a guess, the 17X flight numbers are the original ones whereas the rest are later additions and not added to the schedules in any specific sequence. I think the 175 was always the earliest morning subsonic 747 JFK before an earlier one was slotted in later and numbered as the 117.

Having travelled extensively in the last 25 years, it’s the 178 daylight service that is well and truly in my memory as my preferred return option to London, and always on the 747 before they were retired. Upper deck 747-100 or -200 in the mid to late 90s with the tartan Club World blankets and mugs was my first experience of the 178 but now I am showing my age.