Voltage fluctuates. You could put 9v into it and it will be just fine. Hell you can probably put 12v into it and it will be fine. I have a switch at work that only exists to split one ethernet cable into two, and it's rated for either 5 or 9v, but I'm using a 12v power supply on it and it's just fine because it's power conversion is rated for well over that (I think 16v max).
Disclaimer: don't blindly put higher voltage into a device rated for a much lower voltage. 9v devices will usually tolerate 12v, but not all.