Because you cannot assume that all 10 are qualified.
Based on the USNA's own statistics (let's use the Class of 2012), only 3,838 candidates received a nomination.
Of those 3,858 nominated candidates, only 2,197 were qualified (3Q).
So despite the inflated numbers of applications, nominations, etc - the REAL applicant pool can only consist of candidates who are legally allowed to received an appointment - those who:
1) Have a valid legal nomination, and
2) Are fully qualified.
And in 2008 (Class of 2012) there were 2,197 candidates eligible for appointment (3Q + nom).
Of those 2,197 legally qualified/nominated candidates, there were 1,537 appointment offers.
1,537 / 2,197 = 70% chance of appointment for a 3q + nom candidate.
(For Class of 2013, the numbers were 1,667 / 2,253 = 74%)