I think the difference there (e.g. max dive depth of 1280) is just the 'luck of the draw'  That is, you might have had a deeper theoretical maximum, though the specific dives for that site were shallow.  For example, I uncovered a site via the radar with a maximum depth of 1500 per the table, but the deepest dive that I could do on that site was 1240 (or something like that.)  Just got unlucky as to the specific dives that spawned.

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