Let's do a hypothetical situation here:
All of the players on a KP thread are wanting to fill a level 12 Prosperity Towers, which takes 650 KP. The thread asks for 50 kp from each player.
Let's say a member had 650 points to spend on the 50 kp thread. He hits one player's PT 3x and secures a top chest for 150 points. He hits another player's PT twice and secures the 2nd chest. He earns the third spot on another, with 50 kp, and the fourth on another with 50kp, and a fifth chest on another's wonder. He earns the smallest chest (6th or 7th spot) on another person's PT, with 50 kp. The last 200 kp he spends, he doesn't earn a chest because he's too far down the list. Other members have gotten on there first. So he gets nothing out of that last 200 kp. That's actually rare, but I'm being conservative here to avoid being over confident on odds.
So what did he earn all together? He earned, of course, the 650 kp up front that he got back from players for his own wonder, which is now leveled.
Then, he also earned a chest for 40 points, a chest for 30 points, a chest for 25 points, a chest for 15 points, a chest for 10 points, and a chest for five points (In addition to runes). The rest of the time he got nothing. That totals 125 points. Far more than the 40 that you average. This is conservative, in my experience. I often make more than this, depending upon the value of the wonder.
In any case, I do not average less than I would via the net zero method, and it's fair to everyone. If they don't like the fact that they aren't getting a chest, they don't have to donate. Frankly, I don't care ... I donate anyway whether the chests are gone or not, as long as they still need my points. I figure I'm not losing anything since I'm still getting back my original amount. And I make out well on the chests overall.