Strange altitude reading (deviation)

[Gotcha]

Since I'm grounded on vacation with my Mavic at home, I had time to review the flight recording of my first flight out of beginner-mode. I climbed up to 50 meter, did some turns, reduced height to 30 meters and later to 10. After coming closer to the start point I reduced height (according to flight data) to zero and hovering about 0.5 meters over the start point the reading is -5 meters!
This was about 8.5 minutes after takeoff.
Is an altimeter deviation of 5 meters (~16 ft) within tolerance due to pressure changes or could it be that something is wrong with the Mavics barometer. I'd say this is too much for such a short time, since the weather was fine.
Is the displayed height only a pressure measurement or is it a combination of GPS data, ultrasonic sensors and pressure?

 

[Davzone]

How long did you wait before take off?
If you don't wait for a full GPS position, this can happen more easily.
I usually let it sit for a minute to have stable gps signal and update home point and fly.

 

[Gotcha]

Can't tell how long it was on, but according to the flight log I had 17 Satellites @ take off

 

[Pathogen]

I've read that number of satellites may not matter, as compared to how many of the 5 bars are full on the GPS meter. Also, I like to check the GPS interference detector in the app to find a 'clean' take off point.

 

[Gotcha]

I was on an open field and according to the flight log I had all GPS bars and between 17-20 satellites.
Checking GPS before takeoff would imply that the displayed AGL is somehow derived from the GPS height information, not the barometric pressure sensor