Because this comes up a lot...
Page 84 DMG: "Assuming typical adventuring conditions and average luck, most adventuring parties can handle about six to eight medium or hard encounters in a day. If the adventure has more easy encounters, the adventurers can get through more. If it has more deadly encounters they can handle fewer."
It does not say anything about official game balance requiring 6-8 encounters in a day.
Read more: http://www.enworld.org/forum/showthread.php?473922-Berserker-Barbarian-Fix/page6#ixzz3uIJdmNmA