@Cyber LibertyTexas calls this list, the "Suspense List". California list is called the "Inactive List".
Each county has such a list and names are put on by certain criteria that tells the county registrar that voter is no longer a voter there and the name stays on the Suspense/Inactive list for a set period of time, then the name is removed. It sounds like Los Angeles county has been lax about weeding out voters that are no longer there for some reason - which is usually moved or died.
I looked up the California Election Code and here is their criteria for putting a name on the Inactive List and when the name is removed. It is similar to the Texas law for the removal of a voter name.
California Code, Elections Code - ELEC § 2226
a) Based on change-of-address information received pursuant to Sections 2220 to 2225 , inclusive, or change-of-address information provided directly by the voter, the county elections official shall take the following actions as appropriate:
(1) If the information indicates the voter has moved to a new residence address in California, the county elections official shall immediately update the voter's registration.
(2) 
If the mailings have been returned as undeliverable, or if the voter fails to confirm his or her address as required by Section 2224 , the county elections official may place the voter's name on the inactive file of registered voters who do not receive election materials and are not included in calculations to determine the number of signatures required for qualification of candidates and measures, precinct size, or other election administration-related processes.
(b) The voter registration of any voter whose name has been placed on the inactive file of registered voters for failure to respond to an address verification mailing required by Section 2225 , and who does not offer to vote or vote at any election between the date of the mailing and two federal general elections after the date of that mailing, may be canceled. (c) Any voter whose name has been placed on the inactive file of registered voters and offers to vote at any election between the date of the verification notice, and two federal general elections after the date of notice, or who notifies the elections official of a continued residency, shall be removed from the inactive file and placed on the active voter file.
(d) All address updates, cancellations, and inactive transactions made to the voter registration file pursuant to this section shall be reflected on the voter list as required by Section 2191 .