The Snap execution time only covers the time that the snap was actively trying to process documents. There is still time required to prepare the pipeline and that is added into the Pipeline execution time. Also, if the Snaplex is close to its limit for load, the pipeline can be queued for some amount of time and that is included in the Pipeline execution time. That being said, if you are consistently seeing this amount of difference there may be a performance issue that should be looked at. You might want to contact support and include the runtime IDs for the slow executions.
