But that 21 minute difference is going to be the result of the actual design of HS2 - it's not an automatic result of having an additional stop. Presumably, if Birmingham had been built as a through HS2 station, the design would have been different, so you'd have seen a much smaller journey time difference. Also, I would guess part of that 21 minutes is to allow for the inevitable conflicting moves outside Curzon Street that are the result of it being a terminal station.
Actually thinking about it, I wonder if there's something else. The spur to Birmingham is 6 miles long. Losing 21 minutes over a 12 mile out-and-back along the spur implies average speeds of not much more than 30mph, which is nowhere near high speed. Are the numbers getting distorted by - perhaps - London-Birmingham timings being based on stopping at Birmingham International, and London-Manchester timings based on not stopping there or something?