@Seabass927 is correct that a-g courses and the capped weighted GPA are used for UC admission eligibility. 9th grade a-g courses will be evaluated for completion and rigor since grades and courses are listed on the UC application while the Senior year courses are listed on the application to determine completion of the minimum requirements and also if HS course rigor is being maintained.
I do not know the history of why only 10-11th grades are used for the GPA calculation or why UC’s do not consider Mid-year Senior grades but that is the limitations all students need to deal with when applying to the UC’s.
The UC’s receive a huge amount of applications and as stated by @Seabass927 the 10-11th a-g grades are used to get your foot in the door then the application will reviewed for the remaining 13 areas of criteria.
Remember all acceptances are provisional meaning all Seniors need to complete the a-g courses listed on the UC application with passing grades or they can be rescinded.