Mobs get out of minecart after logging in. (Happens sometimes)

What I expected to happen was that the mobs stay in the minecarts as I log out of the game and back into the world.

What actually happened was that the mobs get out of the minecarts as I log back into the world.

Steps to Reproduce:
1. Put a long railing of any type of rail onto a surface.
2. Put the mob into the minecart.
3. Push the minecart.
4. Log out of the world to the main menu.
5. Log back into the world you have just been in. And as you see, the mob is outside or next to the minecart.