The book says about mastering a psychic discipline:
Once a psyker has purchased 8 psychic techniques from a single Discipline, he has attained mastery of that Discipline, and the Psy Rating for all Fettered Psychic Strength uses of techniques from that Discipline gain a +1 bonus. This has no additional effect on the Unfettered or Push Psy Rating of the psyker.
But the Telekinetic discipline only has 7 techniques, then how can you attain mastery of that discipline? (Telepathy has 9 techniques, and Divination 8)