Ambient light is mostly from lights in the room, windows, etc. Perhaps there is an adjacent room with lights on.
The light reflected from objects surrounding the screen (like a white ceiling) cuts down on black levels, but I have found it is a lot less of a problem than outside light sources. When you have a bright enough object on the screen to light up the ceiling and reflect back on the screen, the bright object tends to cause your eyes not to see the difference in black level.
In a perfectly controlled room there would be no light at all in the room, and all the surfaces adsorb 100% of the light reflected from the screen... Perhaps you even wear ski masks watching the movie to avoid it reflecting off your face... Like I said it is not something most people will notice or care about.
I have one room with dark brown walls and ceiling and one room that is all white. Yes I can easily see the difference with the reflected light. But, 99% of the time I am just watching the TV and do not even think about it. Certainly (to me at least, others might differ) it is not worth the decorating issues of painting the room black.