So you start a load of laundry and the timer displays one hour. Then you come back in an hour to find there's still fifteen minutes left on the clock. What gives?
A lot of it has to do with something called "load sense." What happens is that newer washing machines weigh your laundry before they actually start a cycle. The idea is that the washer will adjust the wash time to save water–the largest contributor to the cost of doing a laundry load. As the cycle progresses, other conditions can affect the time it takes to complete.
If it's a top loader, load rebalancing tops the list of time adders. Load rebalancing is when a washer fills up with water in an attempt to redistribute laundry to get a more even clean or to prevent your washer from shaking. The process is slow and consumes gallons of water.
To prevent this wasteful practice, try washing items of similar density. Do not, for example, wash denim and silk together. If you do, you're going to have a bad time.
Cycles get lengthened at the end of the wash as well. All washers spin as fast as they can at the end of the cycle. This is done to extract excess water from your wet clothes. So if the load sense is activated again and your laundry is still sopping wet, the spin cycle gets extended. Thus, your ten minutes to go turns into twenty minutes to go.
The next time you go down into your basement only to find your washer still running, remember that it's happening for a reason. How does the saying go? "Patience is a virtue."