There is no legal requirement under California statutory law that employers provide severance pay to an employee upon termination of employment. So why would an employer offer severance if it does not have a legal obligation to do so?
An employer may have offered severance when it realized, often through legal defense counsel, it violated employment laws, and decided to low ball the employee to make a lawful claim go away for pennies on the dollar.
Consult with THE Legal Advocate to determine if you should agree to the offered severance, if you should negotiate for more money/benefits, or file a lawsuit.