The automatic mileage tracker starts tracking when you reach 10mph (as well as a few other factors) and stops after several minutes of inactivity. For this reason, one trip may be segmented into several shorter segments or several trips may be combined into one.
If you need to manually stop a trip, you can do so by toggling the Auto Tracker button off, then back on on the Mileage tab.
If you make frequent stops (such as doing deliveries), you may want to consider using the manual tracker. With this option, you can start the tracker at the beginning of your shift, then stop it at the end. All mileage driven in that period will automatically be tagged as business. To start the manual tracker, first make sure auto-tracking is disabled, then tap the orange arrow icon in the bottom-right of the Mileage screen.