The discount is deliberate I think its every 5th automatic pushed preview. The idea is to encourage you to allow pushed previews. If you have a lot of wishlist cards you could get lucky, but I think the choice of which preview to push is purely random.
A better strategy is to check out each weekend's TGIF sale. 2-3 weeks ago they had a buy 3 and get the fourth card free sale. This is 25% or better if you buy 3 low quality cards and get a crystal clear card for free.