The mileage shown on a request tile in the requests inbox is calculated as a straight line from your location to where the event is - so called "How the Crow Flies".
If you click through to the event, there is a map below the quote information with the actual driving distance in miles.
If you are receiving too many requests that are out of your driving range, you may wish to update your preferences.
It's possible that you have on a Receive nearby requests setting which will mean that we'll occasionally send you requests just outside of your area which we think would be valuable to you.
To check if this setting is on and/or turn it off, follow these instructions:
In the main menu open the Request Preferences dropdown and select notifications
Scroll down to the receive nearby requests feature and toggle this on / off
Press save
Alternatively, to reduce your mileage radius all together:
Open the Request Preferences tab in the main menu
Click on Request coverage
Turn up/down your travel distance
Press Save Changes