I changed the palette because 'RdBu_11' is a " *shades-of-blue : white : shades_of-red* " type, intended for anomaly plotting, so it is inappropriate to the data being plotted here.  So, I just thought this was more like good practice (and hopefully looks ok).

Similarly, I think we should be explicitly drawing as many contours as there are colours in the map, or it can be confusing...  
You're right ...
  * it **does** draw just the same with 25 levels
  * but that begs the question "why doesn't it draw the number of contours I asked for?"

The special catch here is, that you wouldn't expect that problem if you're used to the ordinary matplotlib provided colormaps, because they all do *interpolated* instead of discrete colours.
Unfortunately, this seems to be a fairly obscure and ill-documented matplotlib concept, as I've already discussed it twice with other people who didn't know what I meant.
Also unfortunately, I think it is important here -- as without it, attempts to copy + modify this code may be missing an important part of the solution.

We could expand the comment to explain this extra stuff, but I think it is overkill.  Instead, I was just trying to make it a better usage example.