I am also wondering how DAlt is calculated, but have not found much documentation on it. Did you ever figure out what was going on?
For the record, I am sure that it is the “desired” altitude. Just as Alt, It seems to be relative to EKF Origin, but takes more into consideration then just the waypoint altitude. I have seen it vary, even though I always set a constant altitude value to the waypoints.
I guess the waypoint altitude should be interpreted as “Height above ground level” which in OP’s case might result in a DAlt greater on 30m, depending on the EKF origin and the terrain. This post explains the HAGL Barometer has more weight than LidarLite, to calculate altitude
I would still like to understand DAlt and its calculation better, so if anyone could shed some light on this or point me to a resource, I’d be grateful.