You are absolutely right, they are both made from refine white-processed flour!! Refined white flour being classified as a 'complex carbohydrate' or multiple linkage of glucose units, and yes, gram for gram should have the same impact to the blood glucose readings.
I'm guessing, but, your traditional baking powder biscuit: (a slightly denser carbohydrate containing product than a fluffy-puffed homemade 'less carbohydrates' containing slice of sourdough bread), contains more added source of sweetener, i.e. granulated sugar, (assuming the standard biscuit served at MY local breakfast haunt, it is a biscuit with a slightly sweet taste, no sugar subs used here). So, the difference you measure on the meter, (all things being equal*), represents some sneaky denser carbs, so instead of the standard 15 grams for a slice of sourdough bread, the biscuit actually had many more carbohydrate, say 20-30+ grams per biscuit.
Very confusing, emphasizing the difference in cooks. In my past experiences, I've observed cooks using the standard biscuit mix or other mixes, altering the recipe to add their special 'flavor', most often sugar or salt), as to distinguish 'their' product.
Other factors to impact blood glucose levels, (as to reference, "*all things being equal"),
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