This is a relic of the original system of traits. Originally each trait line (5 core ones) was tied to two stats (Power/x, Precision/y, Toughness/z, Vitality/a, Profession specific/b). x, y, z, a, and b were Boon Duration (Concentration), Condition Duration (Expertise), Condition Damage (Condition), Healing Power, and Crit Damage (Ferocity).
When you invested into a traitline, you increased the stats. So profession specific for ranger was Pet Attribute Bonus. That's basically been replaced by Pack Alpha.