That's because you were meant to, the 7d card would have been thicker (usually by sticking two cards together) so it stays in view slightly longer than the other cards when the deck is riffled, subconsciously the brain notices this, and this it is already in your mind when asked to pick a card.
Shout by Adam K.BlockedParentSpoilers2014-07-13T22:59:43Z
I also thought about 7 diamonds ;)