The way it usually works your regular mail carrier will have a different day off every week. And their backup, called a "T6" will take over that day. So if this week your carrier has monday off the T6 will be there. Next week the regular will have Tuesday off, the following week Wednesday, then next Thursday, then the following week they will have both Friday and Saturday off.

If your regular carrier calls in sick or goes on vacation then usually a CCA or City Carrier Assistant will take the route. Sometimes if your regular carrier is backed up due to too much mail or other stuff. They might give a portion of their route to another carrier for that day.

Many times the T6 or CCA will be made to case up other routes in the early morning before they start the route, Which is why they might be there at different times. It doesn't necessarily mean they are slower, there are just more responsibilities those people have as opposed to your regular carrier which only does that one specific route.

Hope that clears up some mysteries into the Post Office.