Classic case of marketing/sales promising something that ops struggle to keep up with..not the first, not the last time. When a marketing team says "all flights will have this" they really mean "all flights as scheduled will have this" and operational factors will not be taken into account. It doesn't make good copy to write "All flights except those where we need to use a different plane will have DeltaOne".

Truth is though that booking a flight with a particular plane and seat in mind always carries a significant risk with it.