The information you see on the PAS records open to the public aren't the detailed records they hold
older records just give "parish" the newer records give a 6 figure ref number but this is the reference is the centre of one square kilometre so that all finds within that square have the same number.
I obviously don't know but in my experience the majority of detectorists who record give the correct information, those who don't I would think are either detecting illegally and don't have permission to detect that land, don't want the the land owner to know what's been found or are just paranoid.
To many detectorists the importance is on the "thing" to archaeologists it's what the "thing" represents in terms of time and place, nobody cares if you've just found a rare Mary half groat that you've been wanting for 20 years but it does pin that place with usage in the Tudor period. Giving the wrong details is worse than not recording at all IMHO.