Bit interleaving in DVB-T

I am trying to understand DVB-T from its simulink implementation. I was the file commdvbt_tablegen.m In line 26 I see that the rows are reshuffled from [1 2 3 4 5 6] to [1 4 2 5 3 6]. I couldn't understand the logic behind that. I referred sec 4.3.4 in the EN 300 744 V1.1.2 (1997-08) but couldn't find any clue for the same.

I will appreciate any help.