https://www.dmv.ca.gov/portal/dmv/detail/pubs/reg_hdbk/appendix/appendix1f_fees
So I'm not 'knowledgeable' about Cali DMV rules, but, I had a quick look at their site, and the above page, at section .0190 Weight Fees for Commercial Vehicles With a Declared Gross Vehicle Weight (GVW) of 10,000 Pounds or Less (Effective 01/01/2002) (CVC §9400)
Any chance you are mixing GVWR and GWR?
GVWR is the amount the vehicle and load can weight.
GVR is just the weight of the vehicle
(and GCWR is weight of truck+trailer+loads)
A quick look suggests to may that the fees just depend on what the GVR of the truck (ie what it actually weighs, before loads are added) rather than the GVWR which is what it is rated to weigh when loaded.
I'm assuming you don't pull a trailer, so the 26,000 pound commercial requirement isn't an issue. (or if you do, the trailer is small enough the the GCWR (rating for when truck and trailer are both at maximum load)
(Maybe I missed something in your explanation , or meed to spend more time at the DMV site)