To continue to explain much too seriously for a shitposting community: a jack in general is a specific type of flag on a ship, specifically the smaller one that flies at the front. It's traditionally only flown in harbour or at anchor

The British one specifically, though, has history messing things up. It actually pre-dates the Acts of Union (or at least the earlier version of it without the St Patrick's cross does) by an entire century. Originally it really was a jack, only for maritime usage, and Scotland and England continued to fly their own flags on land. By the time the Acts of Union made it an actual national flag, everyone had been calling that design a jack for their entire lives anyway, so it just stuck. Both the admiralty and parliament confirmed that either term is acceptable for official use in the early 1900s