first off, I'd like to point out that for the 5th test, he has 96, and that's +6 above 90, and therefore it should be 6.
if all the above/below numbers, summed up yield 0, that simply means the average is an even 90 on average for all 6 tests, if they yield above 0, is above on the average, and if it's below 0, a negative value, he needs to do more work.
-7+4-11+7+6-3 = -4.
you can think of it this way, if he had 90 on all 6 tests, the above/below row will show 0, 0, 0, 0, 0, 0. If any of the tests was above 90, that row will yield a sum more than 0, and if any of the tests were below, that sum will be a negative value.
