system
Given the following post, respond as if you're commenting on Reddit:
human
[KingArthur2004] in the movie's timeline the western Romans stay in Britain an extra 57 years, into a time when they were effectively limited to just Italy in our history. What were they doing right in that reality that they did wrong in ours?
gpt
One of three things, probably. 

A) The Huns didn't invade or invaded less in the late 4th century (maybe there was a plague that reduced their numbers for a generation or two), which means that Germanic tribes didn't flee/migrate towards the Roman Empire at that time, avoiding the resulting eventual animosity and lootings and sackings until later in history.

B) Someone, somewhere, didn't lose a battle, resulting in more experienced Roman troops still being around or alternatively there was an upswell in patriotism (or money, see option c) that meant more Roman citizens were recruited into the army, thus avoiding the military recruiting a lot of non-Roman mercenaries who would backstab Rome at the first opportunity. Again, this problem of not enough loyal soldiers was probably just delayed rather than mitigated, resulting in Rome holding on for a while longer but still going down.

C) Economy. Someone didn't embezzle as much, or an expensive war wasn't fought (leading back to option b with more soldiers surviving), and the Empire had more money to pay their armies and keep everything in order for a couple more generations. Corruption would always have the potential to take down the economy later, but it might have been delayed a few decades if the right group of people weren't grafting the system at the right time.

Or maybe something else. But it usually boils down to economy or skilled manpower.