It's because allowances aren't rolled over. If you complete the bill cycle with .85 gb left, that amount is gone then as well. When you changed the plan, it prorated the plan on both sides of the change. I don't really like that either, so that's why I said I backdate or future date changes rather than change anything on demand. To me it feels a little like buying $45 worth of food at the grocery store, giving them $50 and being told, well, this is what you gave us and we don't guarantee anywhere that we will give you change...but the industry standard now is generally no rollover minutes.