From: Georgetown, Texas, USA
Yes, there is a reason.
In WITP all TF movement occurs in one jump, and reaction is only from the end of the jump.
In AE, TFs move one hex at a time and reaction is checked for each hex of movement.
Having said that, there are still some issues with reaction. Many, many discussions during development on this one.
If a TF has a max reaction of 6, should it only react six hexes FROM THE INTIAL START POINT, six hexes FROM EACH HEX ALONG THE TF PATH, or six hexes FROM THE CURRENT POSITION OF THE TF (INCLUDING PRIOR REACTION).
Many people want to limit the reaction to six from the original reaction point, but the issue becomes less clear as the TF reacts. What if, for instance, the TF reacts 6 hexes toward a big fat enemy transport TF. But, at the same time the TF is reacting, the enemy TF is moving. At the end of the 6-hex reaction, the enemy might be one or two hexes away. Should the TF continue to react? Should it terminate reaction based purely on distance? Should reaction continue until circumstances make it problematic (fuel, moving under enemy air cover)?
In the end, the answer to all these questions was maybe... The more aggressive the TF commander, the more likely he is to order continued reaction. The more unfavorable the reaction conditions, the less likely the reaction is to continue. Eventually the enemy will be engaged or the reaction will terminate.