The static pressure of a filter changes based on how many cfm is blowing through it (more air = more static pressure)
The static pressure is basically how "hard" it is to blow through the filter if you will.
The 12" Thick filters (11.5" actual) are designed for high velocity such as air ducts, the 6" filters are designed for lower velocity use. As such the 12" filter have lower static pressure vs the 6".
The static pressure of the filter is the main thing that helps the filter have a laminar air flow so a higher static pressure is preferred.
Also 12" thick filters make the hood bulkier.
Some people are successfully using 12" filters though, but it is not ideal.
Edited by sandman, 11 January 2021 - 08:32 PM.