Thats the pressure in the tank pushing against the primary regulator. The number is nearly meaningless, as it has no bearing on the amount of co2 in the tank. Many of those who keg (myself included) ignore this number, and some don't bother to fix this gauge if it breaks. Yours may be right or wrong, but either way it doesn't matter. As an aside, tank pressure increases with temp, so expect the gauge to read lower after it is filled (cold) vs when it reaches room temp.