Orcs, kobolds, goblins, bandits—what do they share? A preponderance to trust overwhelming force when ambushing adventurers. While orcs may simply jump out and attack, bandits and even goblins would almost always (at least in the 10th Age) prefer to threaten, to gain money without bloodshed. Yet, it's possible to roll an encounter that doesn't include enough of any of these types of creatures and outlaws to make even a halfway decent attack on a well-armed (or even a poorly) armed party. Let's take an example whereby six adventurers, obviously badly equipped but with a camp-watch going, are in the depths of a forest. An encounter is rolled, and it indicates 4 orcs.
Would four orcs likely attack a party that outnumbers them by two men? Perhaps, if they had assurances they could kill at least three or four people in the initial raid. But the likelihood is low—in fact, being outnumbered in this way might very well provoke a morale check from the bloodthirsty orc raiders before they even set upon the PCs. Logically, the DM might simply decide that no encounters of any meaning would occur unless they reached a certain threshold (we'll call this number n) of monsters based on the size of the party (call this x) and the intelligence/cowardice/greed of the monster or bandits in question.
For example, the 2e MM tells us that kobolds will never attack unless they outnumber their foes two to one. This means the threshold for an encounter in which kobolds attack is always at least n=2x. More potent looking parties will increase the value of n, while greedier, more bloodthirsty, or braver monsters will decrease this value. It's out of the question to assume that a DM would need to make recourse to an algorithm every time he needs to check if a group of monsters attacks, however, so these calculations are all hidden and come down to the DM "feeling" what is correct in the given situation. These feelings can be tuned by forethought—if you consider a few factors before hand, you'll be more likely to make accurate and verisimilistic judgements in game.
Here's a quick breakdown to illustrate the idea.
Bandits - don't want to risk their lives for nothing. Generally, bandits will require n=3x+ to ambush a party at rest or n=2x if they have some major material advantage (for example, they've blocked the road and there are archers hiding in the woods).
Goblins - follow the same rules as bandits, but tend to be more cowardly. They will not ambush without n=3x+4.
Orcs - bloodthirsty. They will take their chances with n=x+2 or higher.
If the party appears to be levels 3-5 or well-armed, all previous equations double.
But this brings up another note—for ease of calculation, this ignores all parties of enemies less than n. However, these parties don't disappear. They should still be rolled. If there are patrols or watchmen at the camp, they can perhaps find evidence of the smaller parties watching the camp... or, even better, if these small parties are not destroyed, killed, or driven off, the next evening may see an encounter of monsters with n number. Oh, ho, ho~