Whats missing from this discussion are the dates of the coins found 'above' pulltabs.
If you're finding pulltabs deeper than say Indian head pennies in the same area, then it is sort of a mystery at first glance! But mysteries are there to be solved.
A lot could depend on the soil at the search site.
A site could have a variety of soils; filled-in soil, loose easy digging soil, hard packed soil and rocky soil.
For example, if the area has random rocky areas that could explain why a pull tab might sink lower in the clean soil areas whereas an old coin a few feet away in a rocky area would prevent it from sinking normally over the decades. I've found Barber dimes only a couple inches deep in pebble/rocky soil. At the same area only a few yards away the Barbers were anywhere from 8 to 12" deep in non-rocky soil.
In all the digging i've done over the years, i can only remember finding a few pulltabs deeper than 6". The vast majority were from surface to around 3 or 4".
And those few deep pulltabs were in areas i strongly suspect were filled-in sometime in the past.