I bought Humble Indie Bundle 9, beating the average, and got all the games.
The page states that there are more games to come:
But the average is now above what I paid earlier. Will I get the additional games, or will I have to pay more, to reach the future average price?
Yes, if you pay above the current average, or increase your payment to above the current average at any point, you will receive any additional games added to the ‘beat the average’ tier.
If you look at the text on the image you linked:
Pay more than the average of x to unlock them when they arrive!
This means you’ll get any new games when they are added.
Extra Source: I’ve done it.
Source : Link , Question Author : BlaXpirit , Answer Author : GnomeSlice