When ordering a ship to join another (which automatically forms a fleet) and ordering the other ship to move before the first ship arrives, the second ship with move from its spot and the first ship will join it at its new location to form a fleet, even if this is beyond the first ship's movement range.

Example: 

http://i.imgur.com/U5w9QCZ.webm

As you can see, the first transport moves to the possition of the second, maxing out its movement. But when the second transport has completed its movement, the first transport teleports to join the second, forming a fleet at the second's location.

I believe this is happening because the 'form fleet' code is processed as soon as the order to move is given, but only completes once movement on screen has finished.