The command execution is the time taken for the spark to process the task. However, after execution, the platform has to gather all the processed data, collate it and then push it for display in the jobs run result. So there should be some time difference between the execution time and job duration. A job has to go through a sequence of steps to do the execution and complete the job
Simply say,
notebook cell/command execution time = the time taken by spark to process the data and perform the task.
The job execution time = the time taken by spark + additional steps for the platform to start+process+complete the job.