It surprises me that according to xg roller++, 13/11 6/5 is virtually tied with 7/5 6/5 after 6155. Normally, 7/5 6/5 tends to be better than 13/11 6/5 on the third roll after the 61 opening. I include a couple of other positions for reference. One where the opponent holds his three point just like he does after 6155 (6153), and one where he has a blitzing oriented position with a good race (and I have good timing for priming...), just like after 6155 (6166).
It seems to me that the blitzing character of the opponent's second roll play might be a factor...
Can anyone help me understand why 13/11 6/5 is so good after 6155 (and 6166) compared to in other 61XX21 situations?
  White is Player 2
score: 0 pip: 147  Unlimited Game  pip: 160 score: 0
Blue is Player 1  
XGID=bEBBcDcebB:0:0:1:21:0:0:0:0:10 
Blue to play 21 
1.  XG Roller++  7/5 6/5  eq: +0,031 
 Player: Opponent:  51,53% (G:15,00% B:0,64%) 48,47% (G:15,99% B:1,16%)  

2.  XG Roller++  13/11 6/5  eq: +0,030 (0,001) 
 Player: Opponent:  52,32% (G:14,06% B:0,60%) 47,68% (G:16,83% B:1,44%)  

eXtreme Gammon Version: 2.10
  White is Player 2
score: 0 pip: 159  Unlimited Game  pip: 160 score: 0
Blue is Player 1  
XGID=bEBBeDbdbB:0:0:1:21:0:0:0:0:10 
Blue to play 21 
1.  XG Roller++  7/5 6/5  eq: +0,083 
 Player: Opponent:  52,06% (G:16,13% B:0,73%) 47,94% (G:14,63% B:0,92%)  

2.  XG Roller++  13/11 6/5  eq: +0,046 (0,037) 
 Player: Opponent:  52,01% (G:14,54% B:0,65%) 47,99% (G:15,40% B:1,13%)  

eXtreme Gammon Version: 2.10
  White is Player 2
score: 0 pip: 143  Unlimited Game  pip: 160 score: 0
Blue is Player 1  
XGID=bEBBcDabebB:0:0:1:21:0:0:0:0:10 
Blue to play 21 
1.  XG Roller++  7/5 6/5  eq: +0,051 
 Player: Opponent:  52,36% (G:15,56% B:0,83%) 47,64% (G:16,72% B:1,29%)  

2.  XG Roller++  13/11 6/5  eq: +0,040 (0,011) 
 Player: Opponent:  52,90% (G:14,42% B:0,70%) 47,10% (G:17,69% B:1,57%)  

eXtreme Gammon Version: 2.10