As the rogue in question, I might argue a few things with regards to Lord Pendragon's take on the encounter:
1) The top of the mountain, where the nest was, HAD to be secured to other reasons, ie the caern itself. So in order to complete the current quest, we had to slay them and/or drive them away.
2) the griffon sighting appened much earlier in the day with a lucky roll (DC 20 or 25, I don't remember). We were down the mountain (let's say a mile or more away) on a rocky hill when we saw them. We knew there was going to be at least 1 griffon on the top (we expected only one mounted by a guard in fact). All characters have at least 5 ranks + dex in hide due to cloak of elvenkind or just plain ranks in hide (two rogues, one monk). Even with a high griffon spot, he would've been hard pressed to see us, since we were trying to pass unoticed due to the expected presence of kobolds in the mountain and due to the suspicious nature of that quest (we had prisoners from the guard of Waterdeep and guards sentinels were sighted).
3) One of the character could fly invisibly to have a better understanding of the apprehended battlefield. We knew that if we didn't studied our targets (the griffon) it might have had dire conscequences on our current mission. So the goal was: as few casualties as possible. So we devise a plan. It was a thought out plan, not just some beneficial circumtsances.
So to comment on your original post, if all the circumstances were in the PC's favor, it was because we did anything in our power to make it so. Isn't it part of overcoming the challenge?
To adress the issue someone raised: it was not a confined space, it was a nest on a rocky plateau. Large enough to allow a family of 5 griffon to sleep confortably. We were very well aware that only one of them could be slain. We expected to fight the second adult, but we were lucky. XP is not exactly computed per encounter (Our DM may give more details on that is you want) and we never felt cheated by the amount of XP he gave us so far
