Frogboy should probably already know this, since on his 'tutorial' video he mentioned he likes to pick Organized and Adventurer so his Sov's stack will move faster ... and of course I'm sure he likes to, later in the game, buy a mount and other equipment to get his Sov (and his Sov's stack) even faster.
The problem is, whenever the Sov exits a city -- with or without a stack -- or any unit, the most movement points they have are 2.0. I have mounted units to serve as fast responders should any of my unlinked resources get threatened, and occasionally stick my sov too.
The same thing happens if I load a saved game. My sov currently has a movement speed of 5, but if the game crashes and I load from the autosave, he only gets 2 movement points 
I'm guessing they are related.