For an execution, the start time and stop time are recorded directly. The difference between them is the real-world wall clock time that it took to run the test. This includes step delay between steps, data retrieval, storage to the database, internal processing, etc.
For each step, we capture the amount of time it takes to execute the step. The elapsed time is a sum of the execution times for the steps. This excludes things like retrieving and storing data to the database, internal processing, and step delay between steps.
So the elapsed time will always be less than or equal to the wall clock time (difference between start time and stop time).