ah_clem, Mike, and Nack all gave simple and logical explanations. When faced by only 7 checkers in the zone we'd rather preserve our racing lead by covering the blot on the 10pt that is subject by quite a few indirect shots from the split white checkers. The more checkers white has in the zone, the more we should be inclined to anchor up. How many does it take? Everybody knows that 10 checkers in the zone pose a serious enough threat to think defense first. While 8 checkers, the same as in the starting position, is still not a force to tip the scales, 9 checkers is the minimum when we should at least start considering choosing the 22pt anchor at the expense of leaving a blot on the 10pt. The last variant illustrates Nack's point regarding split/nonsplit white checkers.
  White is Player 2
score: 0 pip: 178  Unlimited Game Jacoby Beaver  pip: 163 score: 0
Blue is Player 1  
XGID=baECAeDadbAA:0:0:1:21:0:0:3:0:10 
Blue to play 21 
1.  Rollout^{1}  13/10  eq: +0.060 
 Player: Opponent:  51.52% (G:15.31% B:0.88%) 48.48% (G:14.69% B:0.62%)  Conf.: ± 0.006 (+0.055...+0.066)  [100.0%] Duration: 1 hour 21 minutes 

2.  Rollout^{1}  24/22 23/22  eq: +0.031 (0.029) 
 Player: Opponent:  50.53% (G:14.52% B:0.77%) 49.47% (G:13.37% B:0.55%)  Conf.: ± 0.006 (+0.025...+0.037)  [0.0%] Duration: 1 hour 24 minutes 


^{1} 10368 Games rolled with Variance Reduction. Dice Seed: 90614152 Moves: 3ply, cube decisions: XG Roller

eXtreme Gammon Version: 2.10
  White is Player 2
score: 0 pip: 173  Unlimited Game Jacoby Beaver  pip: 163 score: 0
Blue is Player 1  
XGID=baECAdDbdbAA:0:0:1:21:0:0:3:0:10 
Blue to play 21 
1.  Rollout^{1}  13/10  eq: 0.122 
 Player: Opponent:  47.45% (G:13.03% B:0.73%) 52.55% (G:16.27% B:0.71%)  Conf.: ± 0.017 (0.138...0.105)  [100.0%] Duration: 9 minutes 19 seconds 

2.  Rollout^{1}  24/22 23/22  eq: 0.173 (0.051) 
 Player: Opponent:  45.68% (G:11.87% B:0.58%) 54.32% (G:14.48% B:0.56%)  Conf.: ± 0.015 (0.188...0.157)  [0.0%] Duration: 9 minutes 44 seconds 


^{1} 1296 Games rolled with Variance Reduction. Dice Seed: 79605209 Moves: 3ply, cube decisions: XG Roller

eXtreme Gammon Version: 2.10
  White is Player 2
score: 0 pip: 168  Unlimited Game Jacoby Beaver  pip: 163 score: 0
Blue is Player 1  
XGID=baECAcDcdbAA:0:0:1:21:0:0:3:0:10 
Blue to play 21 
1.  Rollout^{1}  13/10  eq: 0.219 
 Player: Opponent:  45.62% (G:12.29% B:0.66%) 54.38% (G:18.64% B:0.78%)  Conf.: ± 0.003 (0.222...0.216)  [100.0%] Duration: 4 hours 59 minutes 

2.  Rollout^{1}  24/22 23/22  eq: 0.228 (0.009) 
 Player: Opponent:  44.46% (G:11.22% B:0.51%) 55.54% (G:14.91% B:0.62%)  Conf.: ± 0.003 (0.231...0.225)  [0.0%] Duration: 5 hours 36 minutes 


^{1} 41472 Games rolled with Variance Reduction. Dice Seed: 79605209 Moves: 3ply, cube decisions: XG Roller

eXtreme Gammon Version: 2.10
  White is Player 2
score: 0 pip: 165  Unlimited Game Jacoby Beaver  pip: 163 score: 0
Blue is Player 1  
XGID=baECAbDacdbAA:0:0:1:21:0:0:3:0:10 
Blue to play 21 
1.  Rollout^{1}  24/22 23/22  eq: 0.246 
 Player: Opponent:  43.94% (G:11.37% B:0.59%) 56.06% (G:15.66% B:0.64%)  Conf.: ± 0.012 (0.258...0.234)  [100.0%] Duration: 17 minutes 16 seconds 

2.  Rollout^{1}  13/10  eq: 0.280 (0.034) 
 Player: Opponent:  44.63% (G:12.27% B:0.77%) 55.37% (G:20.05% B:0.91%)  Conf.: ± 0.012 (0.292...0.267)  [0.0%] Duration: 15 minutes 46 seconds 


^{1} 2592 Games rolled with Variance Reduction. Dice Seed: 79605209 Moves: 3ply, cube decisions: XG Roller

eXtreme Gammon Version: 2.10
  White is Player 2
score: 0 pip: 179  Unlimited Game Jacoby Beaver  pip: 163 score: 0
Blue is Player 1  
XGID=cECAeDadbAA:0:0:1:21:0:0:3:0:10 
Blue to play 21 
1.  Rollout^{1}  24/22 23/22  eq: +0.125 
 Player: Opponent:  52.65% (G:15.94% B:0.95%) 47.35% (G:12.80% B:0.53%)  Conf.: ± 0.006 (+0.119...+0.132)  [100.0%] Duration: 1 hour 20 minutes 

2.  Rollout^{1}  13/10  eq: +0.103 (0.022) 
 Player: Opponent:  52.47% (G:16.22% B:1.12%) 47.53% (G:14.42% B:0.59%)  Conf.: ± 0.006 (+0.097...+0.109)  [0.0%] Duration: 1 hour 19 minutes 


^{1} 10368 Games rolled with Variance Reduction. Dice Seed: 9069776 Moves: 3ply, cube decisions: XG Roller

eXtreme Gammon Version: 2.10