Why is this the best move?

 Have a look at this simple position:

Without much thinking, I played 24/23, 13/11 here. This adds another builder, splits my back men, looks like the straightforward choice.

However, XG stubbornly recommends 24/21 as the best move. Mine is fine, too (-0.05 worse). But still, why should I not put a builder on 11?

The answer is probably that the builder doesn't help much - I already have the 5 point, 11 is too far away from the four point, and as my 8 point is stripped and my opponent has a stone on 2, building the 7 point in the next move with e.g. 11/7, 8/7 is not an option. Also I underestimated that black has some really great rolls like 55 or 44. XG (on Roller++) also says that 13/10 would be slightly better than my move, because then the builder on 10 actually has something to build - the 4 point.

On the other side, black has made the 18 point, threatens to make the 20 point, so helping one checker to escape is reasonable.

Sometimes the straightforward move is not straightforward.

Do you agree? Any thoughts are appreciated.