Have you ever run into a situation where you've been hiking or biking along a trail, but your Garmin GPS device shows your location as being on the nearest road instead? The Garmin GPSmap 60CSx (and presumably other Garmin models) are set, by default, to "Lock On Road" on the Map Page. So even if you are traveling off-road, your Garmin will try to set your position on the nearest road in the street network loaded into your device.
You can easily fix this problem by setting "Lock On Road" to "Off". This is done by going to the Map Page, hitting the Menu button, and selecting "Setup Map". This takes you to the map setup page where you can change the setting from "On" to "Off":
Now you should be good to go.