Because growbags are designed for planting potted seedlings (like they are sold, at 25+cm tall) they don't have to be as finely sieved as MPC. Obviously any bagged medium varies enormously from one manufacturer to another, and price is NO GUARANTEE of quality in any area, especially ones like fineness when they aren't relevant.
If I hear that saw "you get what you pay for" again I'll go ballistic - it's nonsense logic - we've all paid top dollar for things that turned out to be $#!+... The saying that works is "no such thing as a free lunch" it's true, and the shopping version is "you don't get what you DON'T pay for". The other one that's true is that "free" stuff often costs you more than stuff you pay for (but not always).
Growbags have enough fertiliser to "wake up" middle sized greedy plants, but they still need extra feed by mid-season.
MPC should have medium levels of fertiliser for medium plants with medium greed, but not enough to "burn" tiny seedlings as soon as they emerge. The safest way to fit this need is with highish levels of well-aged complex natural fertilisers - but these are expensive.
Cheers.