I did flag this up before to a NowTV Staff Member and wasn't given a full explanation apart from it was the way Fox themselves number the seasons on the files when the content is automatically downloaded to the NowTV server's by Fox.
Here's one thread posted back in 2018.
Perhaps @Jason-C from the NowTV Team would like to expand on this further ?
Thanks for getting in touch.
If I remember correctly, this is related to how FOX broadcast and advertised "Season 1" of Family Guy in the UK. They broke it into 2 seasons.
Since then, all UK seasons are numbered ahead of what the US labels them as. eg. US "Season 2" is UK "Season 3".
That's certainly confusing! FOX directly maintain their catalogue with Sky, so the numbering of seasons and episodes come from them.
For what it's worth, the episodes are also listed as Season 18 on Fox's UK website: https://www.foxtv.co.uk/shows/fox/family-guy#episodes-t18
We'll feed your info back to the relevant teams.