why do they always appear lighter than the original. Example

Original: http://img232.imageshack.us/img232/3226/frame29smallza7.jpg

Facebook: http://photos-d.ak.facebook.com/photos-ak-sctm/v183/220/113/286103752/n286103752_3319595_5493.jpg

Weirdly the facebook is closer to the original scan even though it's taken from that jpg. Weird.