For one, it's a reference to the Constitution: "We the People of the United States, in Order to form a more perfect Union, establish Justice, ensure domestic Tranquility, provide for the common defence, promote the general Welfare, and secure the Blessings of Liberty to ourselves and our Posterity..."

Second, it's a much weaker statement. Compare "we are [imperfect], but that doesn’t mean we are striving to form a union that is perfect." with "we are [imperfect], but that doesn’t mean we aren't striving to form a union that is perfect." The former can be reshaped to we are imperfect, therefore we are trying to form a union that is imperfect---it leaves open the possibility that you wish to make the union worse. The latter becomes we are imperfect but we are trying to form a union that is perfect.

It's probably a typo.