A lot of head scratching and speculation around this one.
The network of 21 Bitcoin Computers does not work like a typical mining pool, so these concepts do not apply. It is set up to give you a close-to-continuous stream of a small amount of bitcoin for development purposes — rather than a lumpy, randomly timed block reward intended for selling on an exchange.
21 has been mining for quite some time, and this is likely why.
Interesting stuff for sure.