In this case, it's probably using a smaller base distance unit as you zoom in.

Your first map shows a segment of 20km. It probably breaks this up into units of, say, 100-200m for purposes of determining average gradients etc, for the sake of not overloading on data points. The largest elevation difference between the start and finish of any of these 200m sections is maybe 25m, giving a gradient of 12.5%.

On your shorter segment, only 2.5km in length, this segment can be broken up into smaller data points, as there are less of them. So for a 20m piece of that section, there might be a 4.8m change in elevation. Thus giving a 24% gradient for that 20m of road.

The 24% section probably only reaches 24% for a short distance. You maybe rode 20m at that gradient. The 20m immediately before might have been 10% and the 20m after only 8%. Stretch it out to 200m, and it would have averaged about 12.5% for that bit of road.