Cron jobs not running on time

on linux redhat, ubuntu systems, what reason would there be for a cron job not running at exactly the time it was scheduled to run, everytime?

meaning, if i put something in cron to run every 120 seconds (2 minutes), why is it that some times, i find that cron would sometimes run the job a few seconds before 120 seconds or a few seconds after 120 seconds?

and lets say cron runs the job many seconds after 120 seconds, what symptoms will this be indicative of?

(this is assuming of course that the cron job was not commented out)

Having cron start a job early would be unusual (as in should never happen).

Having cron start a job late usually means that the system was busy doing something else at the scheduled start time.