I made some similar tests with an airflow meter we use to test aircraft gasper flow. My findings are similar to yours.
I have made some changes to the design to insure all possible airflow is routed through the activated carbon bed in the Chamber. In summary I have added a second flange in the chamber below the flange that the HEPA filter sits on. I have included a Baffle that can be inserted or removed but held securely in place in the slot between the two flanges. The Extended Grate now fits into the Chamber at the aft edge of the Baffle thus capturing all the carbon pellets forward and insuring the area of the Chamber having pellets is completely filled. Finally I removed the top several rows of holes in both the Extended Grate and the Chamber to Fan parts. This will insure that all airflow is forced through the carbon pellet bed even if the area is not completely filled.
Part of this update (not related to the Baffle issue above) is the addition of two new fans, one that is virtually silent and another that has considerable more airflow and higher static pressure. These have been added to address other comments relating to noise and/or airflow.