Maxperson
Morkus from Orkus
It's not. Per the DMG, the rules to play the game are in the PHB. Everything in the DMG is a guideline. Well, except for the alternative PHB rules, but those are not active unless the DM activates them.Given that a lot of the adventures (some written by the designers themselves) don't follow those guidelines, I think it's pretty safe to say that it's not a hard and fast rule.
That said, the designers know the game math and they would not have put in all those numbers, including telling us what the expected amount of xp per game day way, unless the game balance math backed it up. It's probably the strongest guideline in the book.