Cave walls are a lot more irregular than the ocean or a lake bottom. The beam of the sonar is not a lazer, it's a cone. When the structure is irregular and of varying densities and reflectivity like a wooden shipwreck with a mast sticking up, on a fishfinder/sonar you will see a "hard" structure with a "fuzzy" or soft top in the less dense colors. This unit has no way to show (eg) that portions of the beam are coming back from what looks like 30m but the majority of the beam is taking longer and returning from 35m away. So instead of getting to see that there is a hard structure at 35m with some sort of less dense structure sticking up 5m (cabins, masts, kelp) you get a changing depth based on the last few returns and your exact hand position. 35m then wait a second, now 32m, a few more seconds 34m, a few more then 31m etc
For mapping caves it worked best up to maybe 10m. Beyond that the cone is too wide and you aren't sure where its actual reported measurement is since there no spot on the wall.