First thing that comes to mind is... Color separation negatives through the blue filter have to be developed longer than the other separations because blue light results in lower gamma. (That's an explanation I've read, others may corroborate or dispute). Maybe the light on the white towel had more blue than the cream wall.

Second thing that comes to mind is... 15% isn't a great difference, I occasionally have that much difference in test results that are supposed to be the same. (Of course something always turns up to be at fault, like trying to re-use a tray of developer)