Well sounds like to me it's rated for a sustained 500w, but can burst up to 600w if needed. You don't often see that anymore, but it's just giving you a little wiggle room.
However, that's a pretty crappy PSU. If possible, you should return it and get something like a Corsair CX 430 or Antec 430. What is your system?
That would be really nice, but I build the system, it powered up and everything, but it kept freezing, turns out, it had 2 bad DIMM's RMA'd it back too newegg and am awaiting a replacement, the mobo is a Gigabyte GA-870A-UD3 rev 3.1 hopefully the next one will be okay. .