This is the difference between Apparent Power (VA) and Real Power (Watts). It has to do with the fact that systems with power supplies lacking power factor correction put a higher "apparent power" load on a system. For systems with good power factor correction, the difference is often of little significance. A modern Active PFC PSU in a computer will have a Power Factor from about 0.94 at low load, to 0.99x to full load, so the apparent power might be 20-25 watts higher in a system at full load vs. its real power.
Many modern UPS designs provide a large amount of overhead for apparent power, and as such, they get rated that way. The UPS you noted indeed is only rated for 500 Watts of real power, but it has a lot of overhead for Apparent Power.
That large difference between RP and VA is indicative of your standard budget UPS producing a square (or quasi-square wave) output. You'll notice as you get to Line Interactive or Online UPS systems with a Pure Sine Wave output, the numbers start to get very close to each other. You'll also notice they cost many times more.