Have ever taken a shot in Manual mode of the same setting, identical camera settings, but had very different exposures?

Please look at my posting for an example. I am very confused, but maybe an explanation will teach me something new.

Interesting, I'm a photo major in college and I'm not even sure. If I had to guess I would say that light isn't constant and it can vary dramatically without any apparent reason. Just think of the journey light take from the sun to here, allot of stuff can get in the way and change the light quality before it get to us. Hope this helps solve the mystery.