I think flymrpuff got it exactly right.

I may have a little (only a little) less sympathy for your seatmate than some others. From my limited experience, most people flying up front generally prefer aisle seats. Window seats generally seem to be the last F seats selected. If he had a choice and choose the aisle seat, he lost priority in whether the shade was raised or lowered. If I want to see outside or if I want to read by natural light, I'll try to get a window seat. If I am more concerned about being able to get out a little quicker or to get up and move around without disturbing my seatmate(s), I'll choose an aisle. I suspect (but do not know) that he had the same option.

However, I don't think it's fair to suggest that someone from a significantly different angle could determine the intensity of the outside light reflected on the screen, other than direct sunlight. The screens are pretty poor to begin with and worse in some positions than others (as SeatGuru sometimes notes).