BMP 280 exact Altimeter readings
The tutorial sample source code has the line:

Serial.print(bmp.readAltitude(1013.25)); // this should be adjusted to your local forcase


That 1013.25 value should be the local pressure at sea level in hundreds of Pa. If want to show altitude above ground level, you will need to know that for your location and write your code to calculate the altitude offset.

The sensor can't know location -- it only measures pressure. The AdaFruit library provides the altitude conversion, but being generic, it relies on several assumptions.