I would guess that your intuition correctly adjudged 21pt + 13pt is often the better static defense, but that you ignored or underestimated the timing factor... that after 18/13 in the feature post Blue will almost certainly break his midpoint before White has to break hers, assuming she rolls no doublets. This timing contest can be measured by counting the number of spare pips that each side can play.
In the likely event that White's midpoint outlasts Blue's midpoint, Blue puts up relatively feeble resistance from the 21pt compared to the 18pt.
Admittedly, "no doublets" is a weighty assumption. Once White has reduced her midpoint to four or fewer checkers and subsequently rolls some medium doublets, Blue can put up somewhat stiffer resistance from the 21pt than from the 18pt.
In each of the three positions diagrammed below (where Blue trails by 21 pips as in the feature post, 42 pips, and 63 pips, respectively), Blue has 33 to play. (The cube being on Blue's side removes cube actions from the equation.) He has a choice between keeping the 21pt anchor or advancing it to the 18pt.
In practical play, the timing of the first position is vastly more likely to arise than that of the third position, but I think the latter provides an instructive contrast.
(My post in this thread with "Variant" in the subject header spotlights a different issue. At any meaningful racing deficit, the 16pt anchor is a less desirable option than even the 21pt.)
Hope that helps.
Nack
        
 
         
 | | White is Player 2
score: 0
pip: 123 | Unlimited Game
Jacoby Beaver | pip: 144
score: 0
Blue is Player 1 |
|
| XGID=----BBC-B--AeC---c-cbBb---:1:1:1:33:0:0:3:0:10 |
| 18pt is better |
| 1. | XG Roller++ | 21/18(2) 13/10 6/3 | eq: -0.177 |
| Player:
Opponent: | 35.08% (G:1.24% B:0.02%)
64.92% (G:1.41% B:0.02%) | |
|
| 2. | XG Roller++ | 13/7 11/5 | eq: -0.227 (-0.050) |
| Player:
Opponent: | 33.23% (G:1.54% B:0.04%)
66.77% (G:2.35% B:0.04%) | |
|
| | White is Player 2
score: 0
pip: 123 | Unlimited Game
Jacoby Beaver | pip: 165
score: 0
Blue is Player 1 |
|
| XGID=-----BBB---BeE---c-cbBb---:1:1:1:33:0:0:3:0:10 |
| 21pt and 18pt are equal |
| 1. | XG Roller++ | 13/10 11/8 7/4(2) | eq: -0.459 |
| Player:
Opponent: | 24.19% (G:1.65% B:0.07%)
75.81% (G:4.49% B:0.06%) | |
|
| 2. | XG Roller++ | 21/18(2) 7/4(2) | eq: -0.459 (0.000) |
| Player:
Opponent: | 23.57% (G:1.44% B:0.03%)
76.43% (G:2.81% B:0.03%) | |
|
| | White is Player 2
score: 0
pip: 123 | Unlimited Game
Jacoby Beaver | pip: 186
score: 0
Blue is Player 1 |
|
| XGID=------C----BeH---c-cbBb---:1:1:1:33:0:0:3:0:10 |
| 21pt is better |
| 1. | XG Roller++ | 13/10(2) 11/8(2) | eq: -0.606 |
| Player:
Opponent: | 23.11% (G:2.22% B:0.08%)
76.89% (G:16.96% B:0.29%) | |
|
| 2. | XG Roller++ | 21/18(2) 13/10(2) | eq: -0.695 (-0.089) |
| Player:
Opponent: | 16.96% (G:2.20% B:0.02%)
83.04% (G:11.50% B:0.11%) | |
|