MaxAlt,
I'm presuming that you are talking about 320KIAS? This has absolutely nothing to do with your 60 minute diversion time, that time is based on your initial cruise altitude and driftdown and should give a figure closer to 430nms to your diversion airport.
Is this fair????? Probably not as it will take you more than 60 minutes flying time to your diversion airport, but as the airlines plan is to keep you on the shortest route, it is their prerogative to cheat.............
Mutt