Can give you a couple examples that I have stumbled across;
1 - Stickers (Panini, Topps, O~pee~chee etc) I know you asked about cards but the sticker collections are also listed on this site. So when you would place the sticker in the album usually the back gets thrown out. What I have seen looks like the scans were taken and cropped from the albums. I have a bunch of doubles so I was able to add as many of the backs as I could in this situation and was also able to replace some of the scans where the cropping wasn't so good. On my profile under items added - Backs = 2,735 ... Fronts = 1,925 (a difference of almost a thousand).
2 - Blank Back Inserts fronts in a lot of cases have either the same design as the base set, or one of the parallel inserts. So in this case someone loads up the fronts of either the base or regular parallel card that isn't an actual blank back card. Then there are cases I've seen where the opposite happens. Someone will get a blank back insert and scan the back to all the cards in the set on the site. LOL it looks kind of funny because all the backs look exactly the same, where the fronts are all over the spectrum in terms of brightness or contrast.
"They've done studies, you know. Sixty percent of the time it works every time." - Brian Fantana from the movie Anchorman