Apparently jungle creeps accumulate gold and exp somehow and eventually receive a Bountifull Treasure buff, letting you know they've maxed out. How does this work exactly? The more details that can be provided in the answer the better, pretend I started playing yesterday and educate me.
As of season 3, the jungle gold banking no longer happens. From the Preseason 3 Patch Notes:
The “Banking” system that caused jungle monsters that have stayed alive for a long duration to have slightly increased rewards has been removed
When the changes to the jungle were initially made, I did a bunch of tests and wrote a couple of articles outlining the exact growth of one of the jungle camps in relation to the "banking" system which is what the Bountiful Treasure buff indicates.
The short summary is that each camp in the jungle will begin to accumulate a little extra experience and gold value after being left alive for a certain amount of time. The accumulation is designed to encourage ganking by reducing the amount of XP and Gold lost by being out of the jungle, as stated by Morello. The exact data for this banking system has been recorded for the wolf camp as an example in the previous article I mention.
Here are some of the key points regarding the "banking system" or Bountiful Treasure buff:
- A camp does not experience any banking until 1 minute after it has spawned, or after the game time reaches 4:00.
- It takes 1:20 for a camp's bank to become full which is 2:20 after its spawn time.
- While the banking system does indeed reduce the xp and gold lost for time spent out of the jungle (i.e. ganking) when compared with the old system, it is always more efficient to kill the jungle camps as soon as possible.
- For example, if you kill the wolf camp one time without any "bank" and a second time as soon as the bank is full, you get an extra 35xp and 6g compared with simply killing two unbanked camps. However, in the time it takes you to do this, you could have killed three unbanked camps for an extra ~128xp and 44 gold (and the fourth camp would be halfway to spawning).
Basically you've already described what happens. When the jungle camps spawn the gold and exp rewards are set to a certain amount. If no one kills them for a while, the reward on the biggest monster in the camp goes up gradually over time. When you finally do kill it, your gold and exp gained will be higher. At a certain point, the reward doesn't increase anymore and the Bountiful Treasure buff is shown on the biggest monster.
The point of this is so going to gank someone and not getting the kill doesn't hurt your jungling too much. If the gold amounts didn't change, going for a champion kill and failing would reduce the gold you would end up with by a lot and it would be more risky. This way, it's almost always worth it to try ganks frequently (depending on your team situation and the champion you're playing).
More info from Morello can be found at http://na.leagueoflegends.com/board/showthread.php?p=17938707#17938707