"Mastered - You gain 10% offense and 5% defense. Increase buffed ally's movement speed and cooldown reduction by 10%."
Can't say for sure the reason, but pretty sure transfer is giving the 5% def for each person I buff, with 6 people and having the ST "United Strength", 1.5% each person, I hit 80% def and can stay that way.
Also, I'm not gaining the 10% to any offense stat like the jutsu says. Though do get the 1.5% still from ST.