Distance is the horizontal, over-the-ground distance (as seen from straight above) — the great-circle length of each straight leg between two points. The extra length gained by walking up or down a slope is not included, so on steep terrain the real travelled distance is somewhat longer.

Ascent and descent are read from the 3D terrain: each leg is sampled roughly every 100 m and every rise and fall along the way is added up, rather than just the height difference between the two clicked points.

Accuracy depends on zoom. Elevations come from the terrain tiles the map has loaded for your current view. Zoomed out, only a coarse, smoothed version of the landscape is available — peaks and gullies are flattened, so ascent and descent come out too low. Zoom in on the stretch you are measuring to load the detailed terrain, then read the numbers.