Because the main air bleed controls when the booster starts.
Get it too big and the booster starts too soon. So you are pulling fuel from the booster, and from the T slots and depending on emulsion and idle air bleed/idle feed restriction you still are pulling fuel from the idle circuit.
You can’t unfuck that with an idle air bleed change.
Thats why I keep saying all these carbs coming with an assload of emulsion (because somehow in the 1990’s when all this bullshit started it became “orthodoxy” that emulsion was to add air to the system and that’s not what it’s for) and big main air bleeds (I won’t even get into transfer slot length and width) are straight garbage and need to be fixed.
You ain’t fixing all that with an idle air bleed change.