[quantum_mechani]

The rule is that units can't retreat into a sieged fort, this prevents armies from other provinces getting inside the fort even though they couldn't break through the siegers. Since an army attacking from inside a fort can't be distinguished from one trying to break the siege from the outside (infact such armies combine if they attempt on the same turn) there is no easy way to make a special case where they can retreat back inside.

It doesn't completely make sense the way it is, but it's very unlikely to get changed.