Greylock said:
Basically, battles can have favorable or unfavorable circumstances for the players. Now CR never changes. A large air elemental is always a CR 5 encounter. But, if the circumstances are extremely in the PC's favor or disfavor, then it might be worth more than a typical CR 5 encounter or less than a typical CR 5 encounter.
Say, the floor is slippery and everyone has to make DC 10 Balance checks every round or fall down. Suddenly that large air elemental is a lot harder than if you fought him in a "normal" room. It's still CR 5, but the DM might deem that the party overcame a challenge that was more difficult than a CR 5. Thus, he might assign a +20% XP bonus to the encounter based on the circumstances.
In a second example, say the party finds themselves facing a troll (also CR 5) below them in a pit with only simple ropes to climb to get out, and he has no ranged weapon. He has to keep making climb checks to get up, the PCs can start cutting ropes, and shoot him with arrows. It is doubtful that any DM is going to award full xp for this encounter.
So, the encounter is going to modify XP. Note that even if PCs fail to take advantage of the situation or if they expend resources to overcome it more easily, the DM should still take these considerations into account. If the PCs in the first example all used potions of fly or cast levetate or what have you, they should still get the bonus because they used resources to overcome the problem. It was still there, and they overcame it like any other challenge. In the second, if the PC barbarian jumps into the pit with the troll, he should not be rewarded for doing so - that's just stupid. They should still take the lower xp award.
Sometimes the PCs can likewise turn a normal situation into an easy one using good tactics or taking advantage of other things. In this case, they should get the normal xp award. They used strategy to overcome an encounter, which is good. So, if in the OP, the PCs had turned a 30' wide bridge into a 5' wide one, or if they had somehow egged the orcs into crossing when they were going to siege the place, then they would have possibly gotten full xp for turning a bad situation into a better one. However, the setup was one which put the PCs at an advantage naturally, so this should be taken into account when determining XP values rewarded.