Grade inflation is rampant. Today's teachers know these kids are scrapping for college admissions and scholarship money. So all (or certainly more) ties go to the runner. In the old days, teachers were more willing to give you the "B" you deserved because it generally would not affect your college admission, or disqualify you from getting a scholarship you desperately needed.
Also, the bottom quarter of today's classes represent a new breed of lows. The kids on that end of the spectrum are generally a mess. This makes the smart kids seem even brighter, by comparison, and also further prompts the teachers to give the bright kids a further benefit of the doubt. "Oh, hell, if I'm giving Derrick a C- just to shuffle him along, how can I give Julie a B+?.....Julie gets an A."