Case back date stamp vs Serial Numbers?

[Fletcher's Dad]

Afternoon everyone, hoping I can tap into your collective knowledge once more as I have two questions relating to the date stamps on vintage Rolex case backs vs the corresponding serial numbers. And my apologies in advance if this has been covered previously.

1) Which of the two timing references should be take as being more accurate to when the watch was actually produced for a 1675 GMT. Ie. case back indicates I 70 (first quarter of 1970) while the serial number indicates as 2,78X,XXX (latter half of 1971). I assume the serial number trumps all, with the case backs being less reliable but I wanted to confirm.

2) Why is it that this timing delta between case back and serial number is so broad with GMT and Subs (and possibly other sports models?) at typically a 1-2 year spread in many instances, whereas with other non-sports models such as DJ's there in many cases is very little to no delta with both timing stamps often aligning.

Thanks in advance!