Doung some tunimg on this car, and noticed the decel settings on the diff go from 29 to 31, with no 30 in between.
You are right. The values 27, 30, 53 and 59 are not shown, but they are actually there. If you increase the decel percent one by one you will get 28, 29, 29, 31. The second 29 appears to be the 30. The same goes for the other ones.
If you set it to 0 all values are available like they should.