What is done (the names of the elements, including levels) is primarily quantitative. That's the base mark of the technical score.
The grades of execution are primarily qualitative.
Some kinds of errors or enhancements are either/or decisions (the skater fell or s/he didn't . . . although even there there are some near-falls that might or might not be counted as falls depending on how "fall" is defined) or are otherwise quantifiable (how many revolutions in that camel spin?).
But most aspects of the GOEs are matters of degree. Was the jump higher or faster than it needed to be? If so, was it by enough to merit +1 by itself, or only if there was something else slightly good about the jump? If it was both very high and very fast, would those qualities alone be enough for +2?
How fast was the spin? How close to perfect was the centering? How beautiful were the positions? Where does each judge draw the line between good enough to bump up the GOE by +1 each for any of those qualities by themselves or just good enough for two or three of them combined to add up to +1?
Many errors are just yes or no, but if yes, by how much? That includes underrotation, which is decided by the technical panel according to an arbitrarily chosen cutoff of 90 degrees, but judges still have the leeway to mark down the element by -1, -2, or -3 just for underrotation depending on how severe it was, in each of their individual opinions. Same for edge changes on jump takeoffs.
How badly did a spin travel? Where does each judge draw the line between an acceptably small deviation from perfect centering vs. mild, moderate, or severe traveling meriting reductions of -1, -2, or -3?
How weak was the fly in a flying spin? How weak were the positions in a spin or spiral sequence? Enough to mark down? If so, by how much?
How weak or how strong were the edges in a step sequence or spiral sequence? How well did it maintain flow or slow down?
All those decisions are qualitative judgments of degrees of quality along a continuum. Deciding where the dividing line is between good and better, or bad and worse, enough to merit another plus or another minus, is going to vary between different individuals even with the best training even if they notice all the same details.
How should an element with several significant positive qualities and one distinct error or weakness be handled? Do the good qualities and bad ones cancel each other out so that the GOE should be 0, or does one or the other prevail to nudge the final GOE toward the positive or the negative side?
Some kinds of errors have specific reductions that must be taken. When those occur, especially the kind that requires -3 GOE, you'll see agreement among all the judges as to the negative GOE. When an element clearly meets the requirements with nothing special about it, you'll see straight 0s. When an element clearly but not extremely exceeds requirements, you may see straight +1s.
-3 and 0 are probably the most commonly unanimous GOEs.
But in other cases, you are going to see differences of opinion based on where different judges draw the line. Usually it will only be two adjacent GOEs. Sometimes a span of two points (e.g., mostly -2 with some -3 and some -1, or mostly 0 with some -1 and some +1) will occur and we can understand all those possibilities. If the span is wider than that, it's probably either because one judge didn't see something everyone else saw (or possibly did see something no one else did) or forgot a rule that applied to an unusual situation, or else there may have been a complicated combination of good qualities and errors that different judges might interpret or balance out in different ways. Those situations are the exception, however.
(Here's an interesting case.
http://www.youtube.com/watch?v=AMqP7_wNS70
The spiral sequence in this program received GOEs ranging from -3 to +1, and everything in between. Just looking at the protocol, it seems that something wacky must have been going on with that element.
http://www.isufs.org/results/ec2008/ec08_Ladies_SP_Scores.pdf
Watch the program and see how you interpret it. How would you mark it, and why?)