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My question relates to lines which in general are covered by Avrukh (2015). However, Avrukh doesn't explain the phenomenon I noticed in the following lines. These lines are classed as Bogo-Indians, but I've kept the Catalan move order preferred by Avrukh.

[FEN "..."]   
1. d4 Nf6 2. c4 e6 3. g3 Bb4+ 4. Bd2 Qe7 5. Nf3 Nc6 6. Nc3 Bxc3 7. Bxc3 Ne4 8. Rc1 d6 9. d5 Nxc3 10. Rxc3

Here out of 271 games in my database there are 160 with 10...Nb8 and 106 with 10...Nd8.

[FEN "..."]
1. d4 Nf6 2. c4 e6 3. g3 Bb4+ 4. Bd2 Qe7 5. Nf3 Nc6 6. Nc3 Bxc3 7. Bxc3 Ne4 8. Rc1 O-O 9. Bg2 d6 10. d5

Here out of 768 games there are 264 with 10...Nd8 and 69 with 10...Nb8. However, the most popular move (434 games) is 10...Nxc3, which leads us to the following position

[FEN "..."]
1. d4 Nf6 2. c4 e6 3. g3 Bb4+ 4. Bd2 Qe7 5. Nf3 Nc6 6. Nc3 Bxc3 7. Bxc3 Ne4 8. Rc1 O-O 9. Bg2 d6 10. d5 Nxc3 11. Rxc3

Now there are 535 games, of which 370 go 11...Nb8 and 146 go 11...Nd8.

Why is there a distinct preference for ...Nb8 in the first and third positions, and a distinct preference for ...Nd8 in the second position?

Avrukh, Boris (2015). Grandmaster Repertoire A1 – The Catalan. Quality Chess.

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  • 2
    Some thoughts, maybe it is not the reason: In all of the positions, White can play dxe6, retreating the knight to d8 gives Black the additional option of recapturing with the knight. After ...Nb8 White does best to clarify the central situation with dxe fxe, after which the knight can return via Nc6 or Nd7-f6. If White does not liquidate the center and plays Bg2, Black can push past with ...e5. The knight mostly returns via Nb8-a6-c5 or Nb8-d7-c5. Also in from d8 the knight nicely controls the light squares in Black's camp, which may become weakened as he thrusts his pawns forward.
    – B.Swan
    Aug 9 '20 at 12:51
  • 2
    It might be related to whether the e4-knight is on the board. In the case of ...Nb8, dxe6 followed by Nd4 is an idea, hitting the e4-knight. In the case of ...Nd8, dxe6 could be answered with ...Nxe6, preparing to pressure a White knight on d4. The thing is, White could play Nd4 whether Black's knight is still on e4 or has been exchanged off. Since if it's still on e4 and Nd4 is played, Black could always insert ...Nxc3 there. So it's unclear to me how things would be more dangerous for Black if he still has his e4-knight.
    – Inertial Ignorance
    Aug 9 '20 at 17:51
  • These comments could probably be made into answers.
    – user24344
    Sep 1 '20 at 6:59
1

While the king knight remains on the board with options to move to c5 or f6, ....Nb8-d7 is an overlap.

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