The only thing I can think of what he could've meant is if the barrels are bored to a larger diameter such as 18.7 ect. It is suggested that fibre cartridges, assuming your F Blacks are fibre, don't seal against the larger bore as good as plastic does, leading to reduced velocity and a possible impact on pattern consistency.
I have noticed people are quick to criticise cheaper cartridges for one reason or another. However, I shoot Gamebore Velocity Black 28G 7.5 fibre in my SP PRO-12 G5 Black! The pattern it provides on the board is literally perfect, seriously could not be any better! And that is with cheap ammo!