Your fishfinder doesn't show you the depth your ball is tracking at either only the distance from the transducer
that 80 foot on the fishfinder ball may with blowback be only at 70, maybe a little more, even though it marks at 80 on the graph. Truth is if you see fish scattered between 80 and 100 feet on the graph they may actually all be closer to 80 feet, just that the ones near the edge of the cone of your transducer mark deeper than they actually are