After some consideration, I believe that the developers probably made a good decision to display ratings instead of ranks in the table. The average ability of players on OGS is different for different board sizes as shown in the rank histogram posted by @S_Alexander. Using the same formula used for the overall rank would be incorrect.
However, the ratings can be useful. They can be used to check that a game is even on a given board size.
If the volatility were also displayed, it would be possible to calculate the expectation value for the result of a match. The volatility is currently available via the API. In the future, perhaps we could specify that we would like a game within a given range of expectation values?