To expand on the above post:

California has a process to transfer a vehicle that is owned by a deceased person without going through probate.  The process is outlined here: 11.185 Transfer Without Probate (CVC §5910 and California Probate Code §§6401, 6402, 13050 and 13100) - California DMV

I am no saying that's what happened here, or if it's even appropriate in this situation, but it may be how the titles were transferred.