What's odd is the left half of the curves are identical, despite the time-difference. The right half of the chart behaves as we'd expect. Any idea why?
You have probably caught the film at maximum (or optimum) development in the larger faster grains, just before the fog starts. The upper scale is just a bit underdeveloped. Thus the upper scale contrast increases.
This is just a guesstimate, but I have seen something like it before.