I want to circle back to the OP. (TL;DR)
There is no disputing or misinterpreting that the ceiling altitude proscribed under Part 107 uses the term AGL (above ground level) that references a fixed maximum altitude relative to changing ground elevations. Launch elevation is not a metric used in the guidelines for a lot of very thoughtful reasons. I'm amazed by the amount of bloviation to the contrary but it's a critically important distinction that DJI systemically fails to address in all of their UAV products without exception.
DJI's programmatic altitude restrictions are fundamentally flawed because they fail to take into consideration changing ground elevation contours.
I flew a 1,000+ acre desert parcel earlier this year. The lowest point of the property was -89ft MSL (mean sea level). The highest point of the property was along a bordering ridge whose highest point was ~1,200ft MSL. This translates to a variable flight altitude of 311ft MSL to ~1,600ft MSL. (Note: I never exceeded a 400ft AGL while following the terrain.)
DJI altitude restriction algorithms assume and operate on the erroneous assumption that maximum altitude should be limited to a fixed launch-point ground-elevation (relative to sea level). Thus they erroneously cap our altitude at 400ft above launch point. That is not what the FAA guidelines say nor was it the FAA's intent.
The reason DJI did what they did is because it's a lot easier and cheaper to implement restrictive algorithms based on gps elevations (and/or relative barometric pressures) than maintaining and referencing an on-board USGS vector map of ground elevations for every point a bird flies "normal" (meaning perpendicular) to changing ground elevations.
Had I launched at the lowest elevation of my desert site of -89ft MSL I would have run the bird to ground as soon the ground elevation exceeded 311ft MSL. Yet that is exactly what DJI engineered for us and it doesn't work. In fact, it operationally fails in every way possible.
The work-around was to launch from the highest elevation of 1,200ft MSL, ascend 400ft AGL to 1,600ft MSL and maintain 400ft AGL as the ground elevations graded downwards. As long as I followed the ground elevation contours and held to 400ft AGL (or less) no laws were broken.
The real flaw was that I could have, without physical restrictions, sustained my flight elevation at 1,600ft MSL until I traveled above the lowest ground elevation point on the property of -89ft MSL. That translates from 400ft AGL (@ 1,600ft MSL) ascension to 1,689ft AGL (@ -89ft MSL) which clearly violates FAA guidelines.
So tell me, friends, why did DJI cap a max flight altitude based on starting ground elevation and fail to restrict horizontal movement as the AGL increases when ground elevations decrease? The FAA violation is no less egregious than ascending above maximum legal altitude restrictions. Yet, flying horizontally over decreasing ground elevations is somehow okay? It certainly carries a greater danger.
DJI's failure to engineer an ability to follow variable ground contours is a huge oversight, and frankly, much worse than fixing a flight ceiling based on launch elevations. Anyone who has performed aerial land surveys knows this. It reflects a laziness inconsistent with the generally good engineering we've all come to expect from DJI.
Yet ground elevation contour following (also called "terrain following" solves all of the arguments and infighting between those who understand the rules, those who don't, and those who are looking for excuses to legislate drone flights out of existence. It solves the problem by conforming our birds to reasonable flight altitudes AGL regardless of the underlying terrain elevation MSL.
The FAA guidelines provides the means to traverse tall obstacles, man made structures, and terrain; and in fact, allows such traversal to occur within a reasonable horizontal vector distance from such objects. Yet DJI has never integrated the ability to overcome such "obstacles" despite the legal means to do so is at their fingertips. DJI has given us collision "detection", but DJI's collision "avoidance", much less terrain adaptation, is lackluster if not completely absent.
As a pilot, I find DJI's failures both objectionably egregious and hugely inconvenient. YMMV.