More about Em

I have mentioned before that I decided to try and learn how to play the ukulele.  I practise every day and I can see slow progress but I am puzzled by the fact that the same chord can be played in different ways, as you can see here in the diagrams.  Why is this?  I still can't manage the four finger version of Em but I am beginning to get more used to the three finger version.
The car was serviced today and the garage washed it for me, it looks great when it's clean.