I'm not too versed in the technological side of it, but as far as I know, it has to do with how the signals are sent from the device. Most pinpointers are VLF, or very low frequency, just like most popular detector models. With VLF, it tends, and as the potential to, go wonky around saltwater and wet saltwater sand, as well as if the ground is mineralised via another source, aside from the ocean.
PI, or pulse induction, is a bit different in its way of sending and detecting signals (again that I'm not educated enough to explain). They tend to go deeper than VLF, and do a lot better in saltwater and other mineralized environments.
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