Yes, an average of just under 2% doesn't sound like much, but most actual rides that have 100' or more of vertical gain per mile are quite hilly. Keep in mind that one is usually doing a loop and there are almost always some essentially flat sections. So typical might be that half the distance is about flat, one quarter is downhill, and one quarter is uphill - therefore the uphill sections have to average 8% to get an overall average of 2% over the total distance. And there are likely to be some significant stretches with grades considerably in excess of the 8% average.

There's a ride near my home called the 'Nifty 10-50' with 10,000' of vertical gain on a 50 mile ride - so an 'average' of only 3.78% as calculated by total gain/total distance. But it seems like it's almost all going either steeply up or steeply down with grades that go up to a bit over 30%.