At it's core, PeakHour is counting the number of bytes reported to it and using that to determine useful information (traffic graphics, usage etc).
If the information coming from the router is not correct or the wrong interface (SNMP) is being monitored, the information being reported by PeakHour may not be what you expect.
There are a few things you can do to determine if the data reported by your router is accurate. If you suspect inaccurate reporting, these steps may help to understand and fix the problem.