What causes the wet bulb temperature and the dry bulb temperatures to be different?

As the water evaporates from the wet bulb, it draws heat from the bulb and causes the liquid inside it to contract.

Step-by-step explanation:

As a sling psychrometer whirls around, the molecules  of water evaporate from the wick.

It takes energy to move the molecules of a liquid into the gas phase, so the water cools down.

As the water evaporates, it becomes cooler than the bulb, and heat flows from the bulb to the water.

The cold liquid in the bulb contracts and moves down the capillary in the thermometer, giving a lower temperature reading.