It's not necessarily true but does happen especially when great big fibre wads and nitro cards are used because for some reason the average person thinks you load a muzzloader like you do a cartridge.
It stems from the fact that black powder produces so much gas and particulates that on leaving the muzzle speed up. That can push the just emerged wad arrangement into the rear of the shot column encouraging it to spread rapidly.
Using thin cards, paper, or an inert powder helps to prevent this.
I've always maintained that the development of choke was first to override the effects of wads in cartridges from this phenomenon but have no proof!
I personally have shot equal volumes and even more powder and missed and taken game to the same degree as any other ratio.
As for my recommendation of 2oz over 90-100 grains of 3f has nothing to do with the mantra " more lead" blah blah but everything to do with experience.
Anyone that understand black powder knows that it gets more efficient with more load upon it up to a point.
If one manages to get one of those sharp eyed birds in range one doesn't want to miss it by skimping on shot for possibly one dam shot of the day!
Not to get into a measuring contest but I have issue with a couple of points,
Powder burning/gas may appear to be accelerating when it exits the muzzle, it's
expanding at the same rate as in the barrel and is why the load of shot or ball is being driven down that barrel along with the unburnt portion on powder, when it exits, near all the driving force dissipates being sent in all directions (path of least resistance) along with any unburnt powder that open air burns (muzzle flash, inefficient load),
the shot has mass and now momentum, at the point it is leaving the barrel the shot (in this case) is set back against the wad to almost a solid block, with reduced driving force and the wad being a
flat light disc it is left behind, the shot is not seen by the eye but the wad is and often misread, I noted this before when some shooters state they missed a given target ahead/behind/over or under because they saw the path of the wad, not necessarily true, most times they stopped the gun but they saw the wad .
( above: flat to oncoming air)
The key elements are
powder load development, correct powder
burn rate for a given barrel length, this can be affected greatly by the weight of the payload to include weight of wad and card, resistance of payload (how tight is it in the bore), this is no different in modern powder loads, optimum burn is what is being looked for, near all the powder is burnt just before the payload exits the muzzle. This can be hard to perfect with black powder as it is not an efficient fuel and burn rate being established by granular size (composition is mostly the same).
As you noted Britsmoothy your experience led to the load you recommended, if it's broken down the powder load development prosses has taken place, without barrel length given. Starting with 3f powder (burn rate), resistance to that burn rate/gas is the 2 oz of shot ( 874 grain projectile plus wad/cards weight), without chrono numbers I would guess that velocity is not high (under 1,150fps) with low muzzle blast, it works in your gun.
I did some testing a few years ago (3,000 shots), most of the best patterns were with lower powder loads with corresponding lower velocity, heavy shot loads did not benefit from the extra shot, fast loads damaged more shot from set back and abrasion against the bore leading to hot core waste, fillers and buffers can have benefits but make load more finicky.
The OP in this case is trying to figure a load for a 42" barrel .750 bore (11 gauge), by bore size it will be a lower pressure load, to me (just my 2 cents) I would start with a powder load from 70 to 90 grains of whatever F burns most completely before the muzzle is reached, tight fitting wad/card combo with shot at 1-1/4 to 1-1/2 oz (546 grains to 655 grains).
Forgot, The OP also noted an average distance to target (Turkey) of 16 yards.
The user will have to test what works best in his gun.
EDIT: Spelling