To be honnest I don't know.

My guess would be it is to allow as much as possible. (so that the superko rule prevents as little as possible.

you can see the progression

1. positional superko prevents the most
2. situational superko allows more
3. natural situational superko allows a bit more
4. ...
5. Repeat of moves (in the same position the same player may not make the same move) allows even more.

I think Repeat of moves is the most allowing superko rule that is possible while it still will make sure a game will always come to an end. (but it can take much longer)

There is a discussion if it all makes even can make a theoretical difference.

maybe sbest to start a discussion in the rules forum on life in 19 x 19 i have seen some old discussions and private emails about it.

But the best way is to start a new one... (see you there :)

started a discussion http://www.lifein19x19.com/forum/viewtopic.php?f=45&t=1479