I got the 5% today on multiple accounts so it may be they're casting a larger net on this one. I also find that if I go a couple of months without spending much on ebay I stop getting many offers and when my buying picks back up I get them almost weekly (for bucks at least).
Interesting. It almost sounds like some sort of algorithmic incentives that somehow show that people that buy seem to buy even more with coupons and bucks. Kinda makes sense as its akin to the impulse buy in retail stores when you see a good deal.