The illustration shows the payload bay doors open.
In the photo perhaps they are either closed, or open with something hanging out of the payload bay, or some other configuration, making the brightness the way it is???
The doors are almost certainly open. Just like shuttle, there are no external radiators on this plane. They are relatively delicate structures that wouldn't survive reentry.
So they have to be inside the doors somewhere. Shuttle had to open its doors within a fixed time, else abort and return, and keep them open while on orbit. No doubt this spaceplane follows a similar profile.
(Also, solar panels. Shuttle didn't use them but if this thing is spending years in space it is probably using solar panels too. They would be mounted at 90* to the radiators, creating a T-shaped structure with different albedos that should be identifiable in even a blurry picture.)
In the photo perhaps they are either closed, or open with something hanging out of the payload bay, or some other configuration, making the brightness the way it is???