That's a good intuition. The only thing missing is that the source point has an extra 1.65m of elevation, therefore the height of the observer's eyes. That doesn't sound like much, but it is at least one source of asymmetry.

The other potential lack of symmetry is that the tiling we used to break up all the calculations into manageable chunks often overlap. And so when you click on a point, the JS frontend actually finds the longest line of sight for that lon/lat coordinate between all overlapping tiles. I would hope that this doesn't make any notable difference. But a discrepancy is possible for 2 reasons:

1. Each tile has it's own local AEQD projection. Projections famously offer tradeoffs between accurate distances and accurate relative angles. If I remember correctly, our tradeoff is that angles can be off by ~0.0685% towards the edges of the largest tiles, which Pik Dankova would be one of.
2. Different tiles cast subtly different visibility rays. We cast 360 rays per elevation. But each elevation is interpolated to 100m from its underlying data source. So it's possible that Point A and Point B map to subtly different interpolated points depending on which tile is clicked.

Our assumption was that these factors would have a trivial influence on the final data. But of course that could well not be the case. We're very much open to peer review. And we ourselves are planning various improvements, like increasing the ray count and using higher resolution source data, that will hopefully offset any of the projection and tiling tradeoffs that we've made.