From dark to light

There are strange things about the Church of St Mary the Virgin, Kirtlington, Oxon. It isn't straight - look at the misalignment of the arches. There's an odd very dark bit (13th century) between nave and choir, with a transeptal chapel one side, and cupboards for robes lining the other side - a vestry in full view of the congregation, in other words! And thirdly, the choir and altar are dark, lit by one large but rather heavily coloured stained glass window, whereas the nave is exceptionally light. For this blip, I'm standing directly in front of the altar looking down through the transcepts into the nave. I'd expect the altar to be the focus of attention, and therefore brightest.