It's all about how much climbing you do. >5000' in a metric qualifies as a hilly ride and is very likely to include some steep stretches, not long, epic climbs, but enough climbing to find stretches that exceed double digits.

BTW, 100'/mile is indeed approximately 2%, but what goes up must invariably go down so all things being somewhat equal if half the miles are "up" and half "down", then the average grade will be 4%.