The linear regulator creates alot of heat dropping the voltage down.
When I measured my '69 gauges, each drew about 1/4-Amp at full scale, so I added a heat sink to the stock case with the regulator inside. In testing with all three gauges at full scale and without the heat sink, the regulator would start to go into thermal shutdown which limits the current output to protect the regulator. It was obvious in testing with a voltage and current meter, but from looking at the gauges, it just looks like they read lower than they should.