Optical illusion

I've taken photos of this building several times because I can't work out why neither my eye nor my camera sees the roof as flat, even though it is. The effect isn't quite so marked from the right side of the building but it''s still there. It's not because of the buildings around, since cropping them out makes no difference. It might be the grille things on the top.

I saw my mum for 7 minutes today. She happened to be on a train coming through Oxford and I happened to have something I wanted to give her so I headed off to the station. Praise for the people at the barrier who allowed me onto the platform without a ticket and without a fuss.