One would guess it's because the other instances were full, so a new one was made, and either more people haven't joined yet, or else there were more there, but some eventually left.
In the case of multiple such instances, some could be attributed to the people leaving idea, or else it's possible the game ran multiple checks on whether it needed to spawn a new instance before it was able to finish processing the first check, and spawned 2-3 new instances at the same time due to lag.
Sometimes I have to find a sparsely populated instance of ESD because I freeze up when too many people are in my instance. It's actually faster to load the new instance than it is to wait for the hard drive to quit thrashing while everything is frozen around me.
I was looking at the instances on ESD and there are some with only 1 player? Why is this?
As the load goes down later in the day, we try to add players moving onto the map to an instance that is near full, and then when an instance completely empties out we close it. This is to keep from having 30 instances of ESD with 6 players each in the middle of the night, when you could have 4 or 5 that are bustling with activity.