39 - 40 definitely sounds like deep large iron falsing like a buried large ferrous object or pipe. You can sometimes figure out if its a pipe by swinging radially out from the "target" and looking for a continuous line of good hits indicating a deep buried pipe or electrical conduit. You can also use pinpoint mode to get an idea of the relative size of the target. Getting a pinpoint tone over a large area would indicate a large buried target.
28 - 30 could be a large but relatively deep piece of aluminum - soda can.
Again use the pinpoint mode to get an idea of the relative size of the target. It is very telling. Practice with the pinpoint on coin sized targets, pull tabs, and then on a crushed aluminum can to get an idea of how pinpoint will sound on larger targets. To improve your pinpointing proficiency you can tape random targets to the "underside" irregularly shaped cardboard pieces and then plop them face down on the ground to challenge yourself into finding exactly where under the cardboard the target is located. Use irregularly shaped pieces and tape them off center to avoid being able to inadvertently cheat on repeat practices. You can poke/mark the targets with from the top with golf tees to see if you were right.
Also, as mentioned above, ferrous falsing targets like nails tend to not pinpoint properly and typically are typically found in the sidewall of an otherwise properly pinpointed plug hole (i.e., if the target were instead a coin you would find it in the plug or relatively centered in the hole).
Since the depth meter is not always accurate and it is hard to get a relative bead on the size or depth of target due to the lack of tonal modulation (if you don't use the built-in pinpointer). You can touch your pinpointer to the ground before you dig your plug to see if your target is relatively shallow or large to tighten up on the plug radius.
Sometimes, especially when you are in a situation where you don't want to be tearing up the site, it is best to just move on if you cannot quickly recover the target. No shame in that and it is really unlikely you are missing out on something worthwhile.
