Why is saving a picture twice in a row producing different binary in Paint

I thought this was a little odd. Open Paint on Windows (I'm using Windows 7) and draw something (anything). Then save as a .png for example called 1.png. Then save 'n' number of other copies straight away without modifying the image (2.png, 3.png,..etc).

I notice that 1.png has a different checksum to 2/3/4/../n.png. 1.png also varies in such (sometimes smaller and other times bigger) compared to the other images.

What is going on?