is the same definition of balance point being used?
Balance point is the point, where if you put the cue on a fulcrum, it would stay level. the weight to the left is equal to the weight of the right.
Adding an extension, front or back, is going to change that. think of a jack with a short handle vs a jack with a long handle.
Extensions have mass and on earth we have gravity therefore extensions have weight. considering balance points in simplest terms of lever and fulcrum...the fulcrum must move to maintain balance if the lever is made longer or more weight is added to lever. Where the weight is added affects how much the fulcrum must move.
Imagine a 10 foot level resting on a fulcrum. the balance point is 5' from the right end and 5' from the left end. now, add two feet to left end. the balance point does not remain 5' feet from the right end, the balance point moves to 6' feet from the right end. now, imagine the same lever and fulcrum, but one end has more weight (like the butt of a pool cue does). The fulcrum is not center, but shifted more to the butt end. if you add length and weight, distance from the old balance point and amount weight is added (and the extra length) does affect how much the balance point moves and which direction relative to original position of the balance point. cues are not uniform weight. Also, when stroking a cue, there are two fulcrums used to balance the cue. The bridge hand and the grip hand. Adding an extension in the middle balances the extra weight more uniformly between these two because it is between the two fulcrums. Adding an extension to the rear (behind one of the fulcrums) means more work is needed by the rear fulcrum and very little by the front to maintain balance. This is assuming that the fulcrums do not move and distance between grip and bridge is definded by the person's stroke.