The MicroPython editor hosted on the http://python.microbit.org website follows a different update cycle to the Mu editor.
Both MicroPython on the website, and the Mu editor have a version of the MicroPython .hex file embedded inside them, and the version in use will be the version that was current when the editor was released.
Mu, available from http://codewith.mu, is maintained and updated on a faster development cycle by the Python community, so new changes and additions to the API will appear quicker in that editor.
The web hosted MicroPython editor at http://python.microbit.org is updated on a slower development cycle, so compared to Mu it may be lacking some of the newer, more up to date features.
The MicroPython version is listed in the online editor when you select Help from the menu:
The online and Mu editors source code is open source and can be found here: