Are those planes staying overnight at U-Tapao or are they coming in from somewhere else before the flight to Sai Gon and going somewhere else after the flight from Sai Gon?
EDIT: Looked it up and it is a Thai airline, with planes based at U-Tapao. As such the times make much more sense.
EDIT2: Shame they don't have flights from Ha Noi to U-Tapao (yet), might be interested.