What is with version 3?

I have started to notice something with ‘version 3’… Have a look:

  • LWJGL3 completely changes the display and input from LWJGL2
  • Python 3 is completely different from Python 2
  • OpenGL 3 deprecates a lot of functions from OpenGL 2

Does any1 know any others? Any what ‘version 3’? Why not ‘version 2’ or ‘version 4’?