Hi, not a stupid question at all! There's two different promotions at the moment; one for the digital zine which reduces the cost to $0 for anyone who already owns the five separate games, and another that bundles the zine and all five games together for the price of the digital zine ($10).
There's no percentage reduction on the bundle for owning previous games, but if you already own one of the 1400 games it's still cheaper to buy the bundle than to buy the four remaining games at full price.
It's not an ideal way of doing things, but it means people who support the development of the games individually get the zine (which has extra material, more GM stuff, and a starter adventure) for free, and anyone who wants the zine now can get the original games bundled in at the same time.
I hope that's made things a bit clearer!