I bought the Humble Bundle but why does it not have Bastion?

I did spend the $8.34

You should probably contact the Humble Bundle support on the site about it, they can probably do more to help than we can.

Maybe the cost of the average went up between you seeing the average, and purchasing? Did you get the other games for the above average price? (Braid, Super Meat Boy, etc.)

Did you spend exactly $8.34? It could be that at the exact moment you purchased it, the average price was $8.35.
I always round up to the nearest dollar.

I also recently bought the Humble Indie Bundle #5 but for me Bastion(And Super Meat Boy) were $8.35. If I'm wrong, I really don't know what's wrong.

did you make sure you found your steam key for bastion, there is a key for SMB Braid etc, a key for Amnesia etc andd then there is one for Bastion.

What you do is, you click the "increase your order amount" link, then enter $0 and press tab.

If you get a message that says "you will not receive the beat-the-average games", then the average went up while you were checking out and you'll have to add a few more cents.