This question is from: https://www.zhihu.com/question/21585475
I thought this question for a long time. And I think the reason is “diffraction”.
To be added my comment.
Wiki：Shadow blister effect https://en.wikipedia.org/wiki/Shadow_blister_effect
A PDF file: Ray Theory Analysis of the Shadow Blister Effect http://engagedscholarship.csuohio.edu/cgi/viewcontent.cgi?article=1075&context=sciphysics_facpub
And this answer: Why do shadows from the sun join each other when near enough?
And this program: Metaballs – Cubic https://www.shadertoy.com/view/XssSzN