It's extremely difficult to find a Lincoln Cent in MS66 let alone MS68, trust me, I search hundreds of uncirculated rolls a year and send my picks to PCGS. I am usually a point lower than I think, but sometimes a resubmission will buy me the grade I need. I have come to the conclusion that these grades are vanity grades, given mostly to bulk submitting dealers, to promote the values of coins in the coin grading service's holders.
The less they grade MS68 the more the values of MS68 coins, and the more coins that are valuable in their holders. Grading services, the top four that is, are good for authenticating and placing a valuable coin in plastic for protection, but you will lose if you play the grading game for profit on a small margin level.
MS is mint state and MS coins are graded by the numbers 60-70 and a cent can tone brown and not be red.