Ok back to the "an a hundred"

So, if you got a perfect score on a test, you, if you were particularly verbose, would say

"I got ninety-nine percent."

"I got one hundred percent."

If you were less verbose, you might say:

"I got a hundred percent."

Now if you were lazy, you might say:

"I got a ninety-nine."

"I got a one hundred."

Now, if you combine the two, using "a" instead of one, you might say:

"I got an a hundred."

LOGIC.