State utility regulators on Thursday put an end to a system that’s allowed investor-owned water utilities including California Water Service to bill customers the cost difference between expected and actual water usage. The California Public Utilities Commission, siding with its consumer-advocate arm, voted 4-1 to halt what are known as water-revenue adjustment mechanisms, which sometimes resulted in unexpected surcharges on ratepayers’ monthly bills. Commissioner Liane M. Randolph cast the lone vote against the proposal.