In the 9. Ne1 KID, White doesn't want to play b4 at all unless there's a compelling reason to do so.  His ideal plan is to play for c4-c5 and Rc1-c7 without advancing too many pawns, since his attack is intended to be conducted with pieces, in particular that Rook on c7.  The goal is a major-piece invasion, not a pawn storm.  Of course, sometimes White has to fall back on pawn advances, for instance if ....a5 and ...b6 are played, but generally he prefers to play c4-c5 supported by pieces.

LeeRoth wrote on 05/29/11 at 13:48:11:

Thanks for explaining me sooo nicely.
Thanks to TN also.

raja wrote on 05/29/11 at 12:57:27:


After 9.Nd2, White hopes to play b4,c5,Nc4.  Black plays 9..a5 to hinder this plan.  You'll note that the Nd2 blocks the Bc1 and leaves the Ra1 unprotected.  So, after Black plays 9..a5, for White to play b4, he must use two moves:  a3 and then Rb1 to get the Ra1 off of the a-file.  This is considered a good trade for Black.  He expends one move (..a5) to make White play two moves (a3, Rb1).  

In the 9.Ne1 variation, there is nothing blocking the Bc1, so White responds to 9..a5, with 10.Be3 and can now get in the move b4 without having to play Ra1-b1.  Play might go, for example, 10.. Nd7 11.a3 f5 12.f3 Kh8 13.b4, etc.  In the classical King's Indian, with attacks raging on opposite sides of the board, that one tempo can make a big difference.