If Gnu likes 15/6 best then you probably have some settings wrong somewhere, as that play is clearly inferior to 15/9, 8/5. You can see this quite easily if you ask yourself, "Will Red hit next turn?". If you make the 5pt, she will only hit with a 3-3, but if you leave the 5pt slotted she can also whack you with 1-1, 3-1 and 3-2.
The really interesting question is, should White be running off the anchor at all? I think the answer to that is no. "When you're ahead, race, when you're behind, stay back" is a good guide, but there is a sort of grey area where, although behind in the race, your chances can be better in a pure race than they are if you try to maintain contact. That area tends to be in the region of 40% chances in the race with the opponent on roll, so it's well worth considering here, but a rollout (1296 games on XG2) demonstrates that is is slightly inferior. This is probably because the straggler doesn't always get safe next turn, failing to get into his outfield with 3-3, 2-1 and 1-1, an 11% chance of course. Running is a very small mistake, even with all those blots in Red's board.
An interesting corollary is that if you had rolled 5-4, then running is definitely wrong! This implies that leaving one man back acts as a sort of pseudo anchor, forcing Red to play most of her rolls without playing from her midpoint which is what she wants to do next!