All readings correspond to your location but are performed using different methods. Everyone has a different precision and has its advantages and disadvantages.
- The first, above, is a obtained by triangulation of GPS (and Glonass if supported) satellites.
- The second is obtained identifying your position (latitude and longitude) with GPS or Wi-Fi location, and querying a web service to find elevation at your position.
- The third is obtained with the barometric sensor device. The measurement is corrected using the pressure of the weather station of the nearest airport.

Each measure is highlighted in green if the measurement is reliable and in orange or red if not.

The second method gives the most reliable measure, if you are at ground level. But it needs internet connection.
Avoid to relay on this method when you are near a drop-off (canyon or ravine). A small positioning error can give large measurement error.
If you do not have internet connection you should rely on the first measure, GPS triangulation, less accurate.
This method works only outdoors and works better under open and clean sky.
If your device has a barometric sensor and you have an internet connection just for a while before start your hike, you should rely on the third measure.
Its accuracy depends on many factors: the distance from the nearest weather station (airport), time variability (when weather is changing is less accurate).

The third measure is also the most reliable if you are indoor in high buildings or to measure altitude difference.