Allright, here goes--
The August one is not actually any harder, in terms of how the AAMC structures the test questions. The reason it may be scored "tougher" has nothing to do with the test creators, with the test takers--it makes sense if you think about it.
Let's say you take the April MCAT and get 65 right out of 77. This may give you a score of 12, and place you at the 90th percentile (meaning that 89% of the people scored lower than you, and 10% or so scored higher).
Now let's say you take the test again in August, and again score 65 correct out of 77. But now, you are competing with a bunch of people who have studied all summer, devoted big chunks of time to review courses, even MCAT camps (like one at UC Berkley). So now, it just so happens that 15% of the people outscore you, and you are at the 85th percentile, which would give you a score of 11. So, even though you turned in the exact same performance, you were "graded harder".
I hope that makes sense. The test is completely graded on a curve (ie, by percentiles, not by percent correct). The test itself is not any more difficult in August than in April, just your competition.