Altitude Inconsistency and EKF

Can someone explain to me how the ekf3 uses the data for altitude?Because I have a f9p giving rtk fixed status, and set for the source of altitude to be my gps but when I arm my drone the altitude keeps fluctuating when it doesn’t even leave the ground.

Is there some other parameter I should be setting?

ps:I am using ekf3 because I am using gps for yaw.