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Eric BrehmNo card is in 'perfect' condition, so perfection is just an abstract ideal that is used as a point of departure for assigning a grade. Defects which reduce the grade include those introduced at the time of manufacture, and those related to handling after a card enters circulation. Defects introduced at time of manufacture include uneven borders, out of focus image, and printing anomalies such as stray dots or blemishes, etc. Defects related to handling (or just age) include corner and edge wear, chipping, tobacco or wax residue, surface scuffing, color fading, discoloration of borders, scratches, creases, stains, ink and pencil marks, paper loss, tears and pinholes, and so on.
The grading standards define how many defects a card can have to receive a '10', a '9', and so on. You start with perfection and then work down. The more defects, the lower the grade. An 8.5 would be a card that has too many defects to qualify for a '9', but not enough to receive a '7', so that puts it in the '8' range. But is has less defects than the average card in the '8' range, so it is recognized as one of the better 8's, and is thus assigned 8.5.
How many cards will receive the new half grades, whether newly submitted or reviewed with an existing whole grade, remains to be seen. Because of the exponential drop-off in the probability of survival the higher on the scale you go, I would expect there will be fewer cards graded 7.5 than straight 7, and many fewer graded 8.5 than straight 8. In the lower grades, the distribution across whole and half grades may be more uniform.