Similar to any cookie or bar recipe that has few ingredients, the dry to wet ratio has a very small margin of error. If you add too much flour, over-mix, or over-bake these blondies, they will likely turn out dry. That's why it is SO important to weigh your ingredients (at the very least the flour) rather than using volume measuring cups. When baking, be sure to check them very frequently! It also never hurts to err on the side of slightly under-baked!