It's a "play" on pay. With aggregates, the truck is paid by the hour/by the load/by the trip. As an example, you load 25 ton of gravel at the quarry, it delivers 15 miles to the job site, and pays the truck $350 to do so. Most aggregate haulers will then pay the driver about 25% of the $350. So if it takes 2 hours to do a "round", the driver just made $43.75/hr. If it takes him 3 hrs for the "round" he's making less per hr, right ? This is done this way so the contractor isn't getting screwed if the truck is tied up in traffic or the driver goofs off. So, basically, the driver is getting paid while loaded only. Also, drivers aren't paid to and from the job sites. If the job pays hourly, then that's a different situation.
Many pay a flat rate per load, so that encourages the driver to hustle, to get more loads. I would bet the driver getting $17/hr only while loaded, the truck is making it's $$$ by the load and the company is "playing" with her pay by doing it this way.
Brokers and truck companies do this all the time. You, the drivers gets .40 per mile but the load is calculated at a flat rate, like $1800 for instance. Actual miles are 1000, but your company says its 970 and pays you for 970 miles.