FR - Travel on horseback - time problems

[Halaster Blackcloak]

I'm running into a bit of a conundrum here. In the PHB/DMG, it says that creatures can travel 1 mile per 1" movement rate. So a human, having a movement rate of 12, can travel on foot 12 miles in one day. Riding horses have a movement rate of 24, which means they can travel 24 miles in one day.

Using the FR Gray Box Set, the grid and text says that horses can travel 10 hexes per day, but the hexes are each 6 miles, so that would mean a horse can travel 60 miles per day - almost triple the movement rate detailed in the DMG/PHB/MM, etc.

Even the Wilderness Survival Guide gives a rate of 24 miles per day.

So where and how the hell did they get the 10 hexes per day rate for horses in the Gray Box? It should be 4 hexes a day, times each hex being 6 miles = 24 miles.

Am I missing something?