This is the result of a slightly stronger correction on the A100 overexposed file. The clipping is not hard-edged, possibly connected with noise issues. Normally this sort of change from blue sky to monochromatic light grey has a distinct boundary. Either that, or the sky really was just light white, and there is no clipping as such. Anyway, it demonstrates a particular strength of the A100 which anyone who has worked from raw will understand.