"Pinholes" (gaps in leaves) are showing the same light source (its underlying shape) through every hole/gap.

The shape of the light source (with anything obstructing it) is the shape of what a "pixel" (single gap) is there, in what you're calling pixelated.

Thousands of gaps mean thousands of it are being shown so it looks pixelated.

https://en.wikipedia.org/wiki/Pinhole_camera#Natural_pinhole_phenomenon

https://www.reddit.com/r/blackmagicfuckery/comments/pjo2cj/the_way_this_tree_shadow_forms_a_grid_pattern/

https://petapixel.com/2012/05/21/crescent-shaped-projections-through-tree-leaves-during-the-solar-eclipse/

You can test this yourself by taking a light and making a shape in it by obstructing it (like taping a coin in the center of a flashlight to make an "eclipse"), then shining it through something opaque with lots of holes and you'll see the same shape repeating projected. And also there's the factor of how your digital camera captures the light in its sensor arrays if you're talking digital images.