I have noticed this with my dryer as well, and I always use the automatic shut off cycle, never a timed dry.  The time on this is an estimation, and can never be taken as an exact.  It will fluctuate depending on the dampness, or moisture that is being measured in your load.

Using the automatic drying cycle will fluctuate in time, just as stated above.

How this works is the dryer measures the electrical resistance of the air inside the dryer. Dry air will have very high electrical resistance. Damp air will have very low resistance. Damp air or low resistance means wet clothes, and a longer drying time.

When the dryer starts to dry the clothes, the moisture trapped in the clothes has yet to be evaporated into the air inside the dryer. When sufficent heat has begun to pull the moisture out of the clothes and into the air inside the dryer, you will see the sensor change the estimated time to dry. The dryer's sensor knows that the less electrical resistance it detects, the longer it needs to dry. Fluctuations in drying time can be seen throughout the drying cycle.

The time you see displayed should only be considered an estimation. If you need to know an exact time the dryer will be done, try switching to the timed dry cycle. It wont dry as accurately as the auto dry setting, but might fit your schedule a little better.