Anyone here like football? Follow the Patriots or the Giants in particular? Then you know what I'm going to say...
For those who don't, here it goes:
In the 2007/2008 season the New England Patriots were on their way to being only second team ever to have an undefeated season. They won all 16 games of the regular season (with a final game against the New York Giants that the Patriots obviously won).
Then comes the Playoffs. The Patriots easily played their way through to get to the Superbowl, and most were acting like they were practically a shoe-in to win it and to become the second undefeated team in NFL history.
Against them were the New York Giants...the same Giants that they beat in the last game of the regular season. They were a wild card team (meaning that they didn't win their division, they just had a good enough record to get in anyway after all the division winners) and had to go through every round of the playoffs on the road...which is very hard.
Well, as you may have surmised, there was a huge upset and the Giants actually won. The Patriots finished 18-1 and did not become the second undefeated team in history.
In the land of D&D we have something similar. Monsters and PC's have essentially won, or at least survived, every fight they've ever been through in order to get to the point where they're at. Essentially, they're like the Patriots going into the Superbowl. They will usually, unless something massive changes, expect to win. Had half of the Patriot's team been killed in a plane crash, they wouldn't have expected to win. Similarly, if the PC's are faced with 100 enemies, they'll probably expect to be unable to win and will choose to retreat.
Barring such extreme examples though, the general idea would be an expectation of victory. Often, as with that Superbowl game, an expectation of victory means that even if the chips are currently down you're expecting that things will turn around. ie- "We've almost brought down the Defender, once he's down the battle should shift in our favor", or "Man, I'm hurting bad, but once the Wizard drops Flaming Sphere then we should be able to shift the tide into our favor".
Because of that, I would find it exceedingly rare to see underconfident enemies, or enemies that will suddenly decide to flee mid-combat. Not that it can't happen, just that it would be rare. If my party just chewed through 4 Hobgoblin soldiers like they were made of tin foil, then I might expect the Goblin Skirmishers behind them to make a run for it. But the reverse? No, because those Hobgoblins will keep on fighting because they thought that those Skirmishers were a bunch of cowardly weaklings anyway.