automatic sensors
"On my GE Dryer, this is a sensor that detects the wetness of the clothes. Once that sensor thinks the clothes match your "more dry" setting, it shuts itself off. It's a thing of beauty. If the clothes are not dry enough for you, then tweak the setting to a little "more dry" for you next run. You will then zone in on the perfect setting that suits your needs. Give me convenience or give me death! "
Other things will throw off the automatic sensor.
Mixing loads, i.e. light cottons (sheets, shirts) with heavy cottons (jeans, towels). Depending on the mixture, the dryer will either shut off when the light items are dry and leave the heavy items damp, or vice versa.
Synthetics (nylon, polyester, etc.) should also be dried by themselves for the same reaason. Even then, I find have to set the controls on my '78 LK to Maximum Dry or it shuts off too quickly when drying these items. Even then. I sometimes have to use one of the 3 timed cycle options (20, 40, or 60 minutes) to completely dry these items. The extra 20 minutes almost always does it.