Disguise! Not as high of a priority for serving as consistency and placement, but once you get a handle on that, you can make even more of a weapon out of your delivery when it's unreadable. I got completely wrong-footed trying to return a guy's serve last weekend when his setup and toss were exactly what he's been using to go out to my backhand, but the ball zipped through my forehand side. Never saw it coming!
What's starting to happen with your variety is that you're probably altering your move to the ball right as you swing at your serve so that you're in a slightly different position relative to the ball at contact. Instead of tossing the ball to a different spot, you're moving yourself to a different spot, but later in your motion so that the receiver can't get any notice of what's in store. That altered move can work for some serves, but even just taking a different swingpath at the same toss from the same setup could be disguising your serves, too. Have fun with it.
I can produce a flat serve or a decent slice by only altering my swingpath, but I need to toss more out to the side to get more extreme spin on that slice. Even if I telegraph that one though, it's a funky ball to deal with. Good observation above concerning the depth of the toss into the court (mikeler). If it's harder for the receiver to see you slide forward more under your toss, it's pretty tough to know that perhaps a kicker is on the way until after you go up to the ball.