Why, in this Capablanca game, was this move considered bad? Regarding this position:

[fen "r2qr1k/pbpn1pbp/1p1p1np1/4p3/1PP5/3P1NP1/PBQNPPBP/R2R2K1 w - - 1 1"]


I have 2 questions:

1)If the game goes 1..c5 2.b5 the engine shows white slightly better but if 1..a5 2.b5 then the engine shows black slightly better.Why is that??

2)In this game Capa played h6 I think to prevent Ng5 but  according to Alekhine this was not a good move. I don't understand why play another move and invite the Knight to g5?
2) In this game Capa played h6 I think to prevent Ng5 but according to
  Alekhine this was not a good move. I don't understand why play another
  move and invite the Knight to g5?


This position comes from the famous game in which Richard Reti defeated the reigning world champion, Jose Raul Capablanca, in New York in 1924.

[fen ""]
[title "Reti vs Capablanca, New York 1924"]

1.Nf3 Nf6 2.c4 g6 3.b4 Bg7 4.Bb2 O-O 5.g3 b6 6.Bg2 Bb7 7.O-O d6 8.d3 Nbd7 9.Nbd2 e5 10.Qc2 Re8 11.Rfd1 


However, in this position, Capablanca didn't play 11...h6 but 11...a5, followed by 12...h6. Despite this difference, the question deserves to be asked because h6 is still debatable. 

[fen "r2qr1k1/pbpn1pbp/1p1p1np1/4p3/1PP5/3P1NP1/PBQNPPBP/R2R2K1 b - - 3 11"]
[title "Reti vs Capablanca, New York 1924"]

1... a5 2.a3 h6?! (2...Qe7!? {to prepare e4})


Alekhine criticized h6 


  This move, which is difficult to understand, is the best proof that
  Capablanca was poorly disposed that day.


Why is h6 inaccurate?

The move h6 is inaccurate because White does not threaten to play Ng5 (He could have done it before, and the exchange of light bishops might weaken the White king). h6 is a move difficult to understand because at this super high level, moves always have a goal. Here we do not clearly understand its purpose. My guess is that


Black is playing a waiting move because it plans to sacrifice a pawn with c5 but waits for a more favorable moment. 
Black does not want to push e4, which is natural, but tends to favor White. 
Black could have played instead Qe7 to prepare e4.




[fen "r2qr1k1/1bpn1pb1/1p1p1npp/p3p3/1PP5/P2P1NP1/1BQNPPBP/R2R2K1 w - - 0 13"]
[title "Reti vs Capablanca, New York 1924"]

1. Nf1 c5! {was that the plan?} 2.b5! {no thanks!} (2.bxa5 Rxa5 3.Nxe5 Bxg2 4.Nxd7 Bc6 5.Nxf6 Bxf6 6.Bxf6 Qxf6 7.Qd2 h5! {and Black has more than sufficient compensation according to Kasparov}) Nf8 e3 Qc7 d4 Be4 Qc3 exd4 exd4 N6d7 Qd2 cxd4 Bxd4 Qxc4 Bxg7 Kxg7 Qb2+ Kg8 Rxd6 Qc5 Rad1 Ra7 Ne3 Qh5 Nd4 Bxg2 Kxg2 Qe5 Nc4 Qc5 Nc6 Rc7 Ne3 Ne5 R1d5 1-0