SP222 NQRKNBBR has [an average success rate for White (%W)] of only 43.3%, while its twin, SP644 RBBNKRQN, has %W of 59.8%.
Is this a statistical fluke or is there an underlying reason? I noted,
Four of the five twins with the largest difference in %W have one SP in the pair where castling is possible on the first move. This indicates that the ability to castle quickly is an important defensive strategy in a difficult position.
After posting those observations, I noticed that the four pairs of twins didn't support my conclusion. Where castling was possible on the first move, White had a significantly better score in two of the pairs, while Black was better in the other two. I decided to take a closer look at the twins from my example, SP222 and SP644.
The following table shows some basic information for the two SPs. The column on the right shows the most frequent first moves for each SP along with each move's score for White. For example, in SP222 the move 1.Nb3 was played 35 times and scored 42.8% for White; in SP644 the equivalent move 1.Ng3 was played 36 times and scored 48.6% for White. The two Knight moves appear to affect their respective SPs in the same way.
I doubt that the difference between SP222's 1.Nb3 score of 42.8% and SP644's 1.Ng3 score of 48.6% is statistically significant. A more promising investigation would be SP222's 1.c4 score of 35.7% and SP644's 1.f4 score of 61.2%. Both first moves involve pushing the Pawn in front of a Rook. It so happens that the chosen Pawn is relevant to the position of the castled King on that side.
In SP222, the move 1.c4 weakens an eventual O-O-O; in SP644, the move 1.f4 makes space for the Rook after an eventual O-O. Is that the reason for the difference in scores for White? More investigation is required to answer that question.