Question 1
Prestice classes are unlike the base classes considered epic at level 11, therefore cant be take till you reach level 21.
Question 2
check if your current ab is correct. in your case it will be BAB 19 (9 lvl figther, 10 lvl weapon master) +1 (weapon focus) +2 (superior weapon of choice) + enchantment on weapon (+1-3 Iron, Adamantium, Mithril) + stat bonus
Difference in AB could be due to:
Change of gear:
switching a Adamantium blade with a Iron, changing Amulet rings gloves belt.. etc etc with gear that gives lesser bonus to str (or dex if your fitness build)
having wards on:
bless, prayer, greater magic weapon and so on meaning any spell that grant + on ab
bull strength (or cat grace incase of fitness build)