Ok, I checked HardCMP what is right. The Liberty Dreadnought HAS actually 20 (!) turret slots, but Discovery uses only 12. The Hardpoints Discovery uses are "HpTurret_L1_01, HpTurret_L2_01, HpTurret_L3_01, HpTurret_L4_01, HpTurret_L4_02, HpTurret_L4_03, HpTurret_L4_04, HpTurret_L4_05, HpTurret_L4_06, HpTurret_L4_07, HpTurret01, HpTurret02" (adapted from shiparch.ini), while HardCMP shows 8 more turret slots, as seen in this picture:
(I used Roman numbers here, because they are easier to draw:P)
You can see that the Liberty Dreadnought has 20 turret hardpoints, too good only 12 are used, otherwise it would be overkill