Would the blend weigh more than the KAAP therefore creating a lower hydration % when using the same 'volume' of water?
What a given amount of flour, or a blend of flours, measured out volumetrically will weigh will depend on the method used to measure out the flour(s) by volume. For example, if you look at the Mass-Volume Conversion Calculator at http://tools.foodsim.com/
, and look at the Measurement Method pull-down menu, you will see that there are several methods listed for measuring out items volumetrically. Now, if you use that tool to determine how much a cup of King Arthur all-purpose flour weighs that was measured out volumetrically using the Textbook method, you will see that it weighs 4.415 ounces. If you do the same for a cup of King Arthur bread flour, you will see that one cup of that flour measured out volumetrically using the Textbook method weighs 4.386 ounces. If you use a different flour measurement method, you will get other weight values for the flours using the different flour measurement methods. For example, a cup of King Arthur bread flour measured out using the Medium method weighs 4.946 ounces.
When water is measured out volumetrically, it, too, is prone to variations. For example, to measure out one cup of water to be technically correct, you should place your measuring cup on a flat surface, and while viewing the markings at eye level, fill it to the desired marking (e.g., a cup). You want to fill the cup until the the lower meniscus of the water is at the desired marking. If that cup of water is weighed on a scale, it should weigh 8.345 ounces. In my experience, when people measure out a cup of water casually without paying much attention to the markings, that "cup" is more likely to weigh from 8-8.2 ounces.
In your case, there were several volume weighings that could have been suspect and not precise, and any such suspect measurements could have been responsible for the results you got. When recipes are stated in volume measurements, it is inevitable that you will end up with imprecise results. To avoid these kinds of results, you have to make adjustments to the quantities of ingredients until you are satisfied. Or you look for recipes where the ingredients are given by weights and use a digital scale to measure out those weights.