Learning a language means having to suspend your own cultural interpretation of the words and the (sometimes incorrect) expectations that go along with your personal experience. Find the nuances of the new language.
I agree with danteRosso, "connects" is a more natural translation, even when Italian has another word for this. "Unites" would mean a million of people feel they belong to the same group and that is more than what a bridge can achieve.
Why the translation "The bridge unifies a million person" is marked wrong? I think unifies is more appropriate than unites. The bridge is not a community, association or a political party , but a structure.