Reliability has to do with how constant one can keep the environment the test is administered in. An administrator would want to have his subjects perform in the same environment, listen to the same type of music, be surrounded by the same people, receive the same type of motivation. Everything must be constant!
The definition of reliabilty is will the testers get the same results if re-tested tomorrow?
Since Pull Ups require a vast amount of Upper Body Strength the chances that a tester improves the day after will be rare. In fact, there is a chance that the tester could be sore from their workout the day before and perform worse.
However, a tester getting the same results is highly likely in this situation, since Upper Body Strength is not something that one can develop overnight.