Could be different rates for different cities, as well as different size of packages perhaps (e.g. upto 5lbs pp 18, 10lbs pp 20 etc..), since it's BYOC (thanks Uber..) it also might be related to your maximum distribution range, the further you are willing to go the more you are getting paid since it increases your fuel costs.

Might actually be a per package/delivery rate that they estimate to be $18-25 per hour. I highly doubt Amazon would pay an hourly wage when some people might only do one delivery per hour. It's just marketing to attract people, just like how Uber advertises to drivers that the average Uber driver income is $90,000/year.

Could be some sort of surge pricing model. During busier times, the rate could go up to attract more drivers.

it could be depending on how many units you can deliver per hour on average ?