Uneven back development

guy’s back muscles and we have seen lots of progress.
The other day the low sun was hitting his back just right and I noticed that there seems to be an area midway between withers and loin that is not as developed as the “fore” and “aft” areas of his back You really can’t see it. You can barely feel it. Looking critically at sweat patterns it looks like that area is not filling up into his saddle. We do belly lifts and I cannot see or feel a “flat” spot.
We have Regular saddle fittings with Schleese rep. Panels are smooth with no lumps bumps or soft spots. Last fitted about 6 mos ago.
Ideas? Is this just how muscles develop in a 15 year old?
Saddle fitting is scheduled.