Answers

Unfortunately, most new homes (particularly those built by developers) are built with little regard to quality and durability, by less-technically-savvy tradesmen at the lowest price point possible to appeal to a prospective buyer's sense of value. 

Should you be concerned?  Most of these sorts of issues are not conditions potentially leading to structural failure but rather finish failures and just generally less-than-desirable quality.

There are many industry specification references for quality, and likely your builder references a spec which is more forgiving of such issues.  For instance, a spec for drywall might say "out of level or line x inches in x feet" or simply "imperfections not visible under normal lighting conditions when viewed from x feet away".  You may want to research which specification may have been referred-to as quality guaranty when the home was sold.

Of course, this is a generalization and does not apply to all new construction.  One would want to engage a building professional to determine what may be happening in the particulars of your home.