Systemic square footage inflation on Redwood City duplexes?

Nearly every duplex I see listed for sale in RWC shows a square footage number that is 40%+ higher than the actual interior living space. For instance, this home shows 2160sqft but the interior space from the seller-provided floorplan sums up to approximately 1200-1300sqft. This makes comparison shopping more complicated.

The inflated number is always listed in the county records, so I assume at some point there was an official policy to count the garage space or something. Does anyone know the history behind this, what the actual policy said, and/or why it was done? Was it a scheme to increase assessed value prior to Prop 13?