altitude difference

I have a mission I planned with 48 waypoints ranging from 0-50m and totaling 1.3km. When I returned to my homepoint, my altitude said i was at 8m when I was on the ground. ive watched the video several times trying to understand, but what could cause this? Would wind be enough?

Hi Everyone New to the Mavic 2 (just traded up from a GoPro Karma) Loving the new tech but just had a question. Taking off from a hill the altitude is set there and when flying down to the ocean I am in the negative numbers. Anyway to set it to sea level? Cheers

Can someone please help me figure out why the jpg below reports to be at 738.59 something when the altitude was approximately 350' AGL? Thank your for your help and forgive me if it is something really obvious and escapes me 'cause I'm old and losing it... Why does it say