Estimation of curvature is notoriously unreliable and depends to a large extent upon the length of the arc perceived. have a look at the three arcs in the diagram below.
The curvature characteristics of the three arcs are actually precisely the same. The shorter lower arcs are just shortened versions of the longer upper arc.
Curvature is perceived by summing information over a significant area, and the larger the area the more accurate the perception of curvature tends to become. The shorter arcs offer less information and the perception of curvature is much less precise.