Can't jump into 1 and a half block slab

If you try to jump and shift over two slabs with a space between them of 1 and a half blocks you can't.

What I expected to happen: Leaving half slab on te floor and putting over it an slab, leaving 1 and a half blocks and putting a secod slab, jumping and getting between both.

What happened: You can't get between the slabs unless you add an extra block on the floor and you stay over it.

 

Here is the link to the video: https://www.youtube.com/watch?v=FApU0FOHqWI&feature=youtu.be