Why use bmp180 sensor to measure altitude is different?

Hello, amazing people!
I used bmp180 to test the atmospheric pressure and altitude and referred to this article about the datasheet. When I tested it at 9 o’clock in the morning, the measured altitude was -11 meters. After a long period of power (about 4 hours), I measured the altitude and the altitude was positive. Around 39 meters, does anyone know what went wrong? The altitude has been rising and the air pressure has been falling.
Thanks in advance!

This was the wrong reading.
If the measurement value is stable then that is the correct reading.
Also, make sure sensor is not a faulty one.

This is normal as the air-pressure locally increases or decreases. 1hPa means 8m (30ft). If the pressure decreases the indicated altitude increases and vice versa.
This is the reason why pilots have to adjust their barometric altitude meter in regular intervals.

Yes, I am sure the sensor is not a faulty one.