In my experience, a team of X player will face X+1 or 'worse' odds about 75% of the time.
Of course, I'm the DM so...
Minions are the most common reason for this numerical inferiority. Here are the two others :
1- Often a DM will use creatures a few levels below the PCs. In 4e, monsters can remain dangerous on the batlefield for quite a few level. There's no shame throwing a few level 1 monsters at level 4 PCs ; you just put more.
2 - 1 standard monster of equivalent level per character leads to moderate, even fairly easy challenges challenges. If you have 4 level 2 PCs and give them 500xp worth of monsters, they won't sweat much. They'll lose a few healing surge and keep all their dailies, typically. To make them work for it, you increase the encounter level one level or two.
Right now my group is 4 PC, level 3 and they are headed for the following endgame fight :
6 kobold minions, 1 kobold slinger, 2 kobold skirmishers, 2 kobold dragon shield and 1 Kobold Wyrmpriest.
850xp, 12 monsters.