On both the autumnal blot and the winter blot the shadow of the higher stone passes across the lower stone 15m away, allowing the shadow to just reach the top of the lower stone. By coincidence the last possible days for autumnal and winter blot occur at almost the same solar altitude.
The animation shows the shadow’s passage the day before, and exactly on the last possible day of the autumnal blot and the winter blot:
The gradients of the top of the stones are in good alignment with the sun’s path.
The shadow from the higher the stone ends up in the right height on the lower stone on the last possible day for the winter blot, one day before 1 1/2 moon cycle after the winter solstice. The shadow ends up one day late on the last possible day for the autumnal blot.
A tree trunk and many branches obscure the sun making it hard to see the shadow as it moves across the lower stone.