All to do with the fare hierarchy and revenue management.

Made up example:
J £1000
C £800
D £600
R £500

Y £820
B £550
H £480
K £400
M £300
O £200
N £100

So that's your fare hierarchy. Then the revenue management people will look at demand on particular routes. On busy business routes they may only open availability in J and C, and the more expensive ET buckets. On more leisure routes they may open up cheaper ET buckets.