[invertedohm]

Exactly. The simplest explanation in my mind is that it's a white balance issue. Most photos are taken in some form of daylight, which is generally in the orange range without a pure white on a "correct" exposure. On average, photos will be taken in warmer (orange) rather than colder (blue) tones.

It would probably even work on a bunch of photos taken of a white wall in some natural light, if you averaged them out. More of an orange hue than pure white, certainly.