"It's not rules as much as guidelines."

The important question is: are the players having fun with the level of challenges you present?

If they're not, the guidelines in the DMG are clearly not working for this party, and you need to adjust some.

Don't think of what's in the DMG too much as "it's in the book so I must follow it!", but more as a toolbox. You take what you need and what you like, to create an experience. I know others have stated it in this thread, but one of the big perks of D&D and other RPGs is that it is incredibly flexible to use for play.