That report is going to show whatever you enter in your taxable miles. We don't automatically calculate that for a number of reasons including:
1. Venue Information is often incomplete. Mapping and distance APIs will give a "best guess" of an address which could be WILDLY off.
2. Business miles for a given event could be radically different depending on where you start driving from (hotel, previous event, etc)
3. People don't drive to every event. (for example - my current gig)
Ultimately the responsibility for record keeping has to fall on you, but we do provide tools to make this easier. When editing a gig you do have the option to calculate the taxable miles for a given event:
If most of your events are simply driving from home to the gig, then back home, the "calculate round trip distance" is probably sufficient. Otherwise you'll have to enter whatever your actual milage was for the event.