Ah that makes sense, thank you.
If you look closely at the frontal bone scan, you can see the shape of the cut to the chin. The bone scar of chin cut is kind of straight horizont below the lower central incisors and it angles down to form sort of a notch below the the nerve holes. So, below the initial cut they make is where they can shorten areas of the chin and/or 'anterior mandible' to adjust for symmetry if needed. It's not uncommon for areas to the central chin and or the anterior mandible (areas beside the central chin) to be a little long or off center. So, when adjustments to symmetry and/or excess length are made to the anterior aspect of the mandible (central chin and area beside it), it avoids asymmetry from the chin being carried through the rest of the cut below (inferior to) and beyond (towards the posterior direction) the nerve holes.
Now, beyond and below the nerve holes, they continue the cut to the back of the jaw and they want the areas released from the cut to be as symmetrical as possible. Basically, they have to PRY areas and PULL down to flare out and lengthen the posterior jaw angle area. Prying away and pulling down are 2 FORCES that could snap the bone if they are also coupled with the need to apply them unequally to adjust for asymmetry at the back of the jaw line. Also, if they have to pry out a LOT to get a large flare out. So, ability to apply those forces to both sides of the jaw segment, somewhat equally, is enhanced by addressing what ever adjustments needed to the anterior mandible (central chin and beside it) to cut down on asymmetry/excess length coming from that area so it does not carry through to the segments they need to pry out and pull down beyond (posterior to) the anterior mandible. In this particular case, I think that is what the doctor was dealing with and describing when he mentioned anterior shortening and centering associated with lateral outward flaring. Like not too much shortening or alteration to the anterior mandible but just what was needed to safely pry and pull at the posterior mandible.
The (frontal) increase in width is an improvement yet is subtle. But that is understandable because too much prying and pulling to effect a more dramatic flare out can be a recipe for an unwanted snap when done in an initial surgery.