What an arch actually is
An arch forms when an object passes through the edge of the cone, then the center, then the far edge. It is a record of geometry, not of species.
A stationary fish under a stationary boat makes a straight line, not an arch. A fish holding tight to bottom may not separate from the bottom return at all.
What fools people
Thermoclines, bait balls, submerged branches, and your own trolling motor cable all return signal. Turn the sensitivity up far enough and you will find whatever you want to find.
The discipline
Believe the bottom, the depth, and the presence of bait. Treat everything else as a hypothesis to test with a cast.