Could be that they had inadvertently slowed and noticed when they saw you overtaking them. They then sped up to the speed limit, not to be a dick to you but to get to the speed limit.
That is a likely scenario. But, as I posted earlier:
And If you're getting passed, then let the other car pass you. You can match their speed once they are in front of you.
And, as @blakeyrat said:
Doesn't matter. They fucked up. They should keep going slow so they don't fuck other people also.