regulator reading on a newly filled co2 tank?

[pocilipora]

Temperature changes do cause the gauges to fluctuate. So after the tank has been inside for a while it should come back up to 800. Ive noticed this at work. Your tank may have been sitting out side.
Just as a side note, a regulator will actually freeze and form frost if a high flow rate of Co2 is used. Or if there is a leak.