yes GPS is often off a little
in the case of my iphone, i used it to find certain underwater structures. I had them marked on GPS when I found them in the late fall a few years ago when the lake was low. As the lake refilled, they disappeared from view (10 feet+ underwater0 but they still hold lots of fish. Anyway, as I went back to them, sometimes I'd wear the fish out, sometimes not. Last year when the lake was low again I went back out there and used my phone to get close. From there I can normally find it visually. Looking at the phone, idling up to "the spot", I then stood on the bow looking around as the boat is easing about. Clear skies middle of the day. I could NOT find it. Finally started going in a spiral and FINALLY (after 40-45 minutes) spotted it about 2 foot under the surface, as expected based on the lake level at the time. Looking at the phone as I parked the boat on top of it, it was off by about 16 feet. Ironically I had just bought a new GPS fish finder and marked it on that. It's much more accurate but it still can only get you maybe 5 foot radius and then I still have to look. 5 foot is close enough on that one, but there are many others I have marked that you basically have to be on top of. Note that these are consumer electronics that don't have to be "that" accurate, and there are much better systems out that that will put me a little closer. For what use these for, they're accurate enough given the (lack of) budget.