Well, when they fill a CO2 cylinder - *any* CO2 cylinder - it will almost immediately go to around a certain point, which is a somewhat variable pressure at which CO2 gas turns to liquid (somewhere around 850 psi at room temp), and then, while still filling (the weight will increase) it will stay at that same pressure.
When used, it's the reverse. It will stay at 800-ish psi until nothing but gas is left in the cylinder, which will be at less than 5% of the total CO2 volume (meaning, for 95%+ of the life of the fill, pressure will stay at 800 psi). At that point, pressure will begin dropping.
What's happening is one of two things, assuming you don't have a massive leak: One, the cylinder was barely filled, and within a couple weeks it will be empty. Or two, your gauge is either broken or off by about 150 psi. In the first case, you need to give the gas store a piece of your mind. In the second case, you can either live with the funky gauge, or replace it.
If it's just off by 150 psi, you'll see when the pressure starts declining and there's no reason to make any changes unless your OCD kicks in (mine would, so no criticism there on my part).
If it's done broke and still reads 650 when the pressure is actually at zero (and the gas is gone), I'd advise replacing it. Probably $15 shipped for a basic brass gauge from Evilbay or a brewing shop. If you have a Milwaukee, Azoo, Aquatek, Dici, or any of their clones, definitely replace it. If it's an industrial single stage reg, there's less worry - you probably won't get any significant EOTD.