Spreadplayers don't spread on blocks

What I expected to happen was ...
I placed some blocks in a completely empty space and used spreadplayer
I expected to randomly get teleportet on one of these blocks.

What actually happened was...:
I got teleportet about 300 blocks in the air the x and z coordinates were different every time.
Describe what happened here

Steps to Reproduce:
1. place some blocks in an empty space.
2. ...Use the spreadplayer command in a commandblock or with the chat both are broken.
3. ...Get teleportet around 300 blocks in the air