Question about subwoofer mounting depth

[DJin TX]

Ok so I have very little knowledge/experience in the CD world of car audio, but planning a system in a few months. As I put together my plan I am just trying be to piece together some info, and am curious about subwoofer mounting depths vs quality of bass. I see the below three subs for Alpine:

10 inch S series (S-W10D4) - not marketed as a shallow mount, mounting depth of 4-11/16

10 inch truck series sub (shallow mount) (SWT-10S4) - mounting depth 4-3/16

So my question is, since the S series 10 inch, and the truck series 10 inch are extremely similar mounting depths (only differ by .5 inch) how is this truck series considered shallow mount, or is it not really a true shallow sub?

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