[pothibo]

I haven't checked the code so I may be wrong but it's possible it's for multi-threading reasons.

[microtonal]

MRI has a global interpreter lock, so that does not make much sense.

In fact, the diagram is simply wrong. This was rectified by the author in an article two weeks later:

http://patshaughnessy.net/2012/1/18/seeing-double-how-ruby-s...