Please provide sample output of that sketch.Your variables are not initialized. Don't expect them to be always 0 when you enter the loop for the first time.The pressure at sea level is not always 1013.25hPa! That value is changing from hour to hour sometimes even from minute to minute.
Here is the output of my bme280 sensor. You can see there is constant variation. This sensor is in a fixed position.What is your project? You may need to use two sensors, one in a fixed, low position.
I need a quadcopter to keep a particular altitude. How i should do that if the values oscilate so much?