That's because many of today's all purpose flours contain baking powder. Using different brands of all purpose flour will give you different baking results because of the protein amounts and other additives.
Is there any way (ideally, of course, reading the label) to tell if packaged flour does contain baking powder, and, if so, how much?
I'd think that by not knowing, one could screw up quite a number of recipes.
How All-Purpose is All-Purpose Flour?
When using all-purpose flour, do you still need to use baking soda, salt, and baking powder when the recipe calls for it?
Yes. Yes, you do.
Perhaps you’re thinking the term “all-purpose” means “all-inclusive,” or more likely, you’re mixing it up with “self-rising” flour, which does include salt and leavening agents. All-purpose flour in this country is our name for regular old flour, with no leavening ingredients, no salt, and not too much protein and not too little. All-purpose flour is called for in most recipes, as opposed to bread flour, cake flour, pastry flour, self-rising flour, etc., etc.
Not all all-purpose flours are the same, however. Some have more or less protein, some are milled from softer or harder wheat, some are bleached, some are bromated, etc., but there’s nothing we can do about that. For all intents and purposes in this huge country of ours, if a recipe calls for flour, you go to the store and select from two or possibly three brands of all-purpose flour. Then you follow the instructions in the recipe for adding the correct amount of yeast, baking soda, baking powder, salt, etc., just as you would for all the other ingredients.