That may be an ingredient to it, but the main reason was to not give so many As (or in our case 9s and 10s).
Actually, from what I remember of all the past blog posts Lazeez had on this before they got eaten by a dying server is a little different. If I've got this wrong, I'm sure I'll get jumped on and corrected by someone. Also, I'm sure the reality is a lot more complex, but this is really a simplified explanation.
From what I remember the history of the situation is what shaped the current system and the key factors are:
1. Provide the readers with a measure of relative worth to other readers about how they liked the stories.
2. Different scoring systems in the past, due to the system evolving since the site started. Of which there has been four or five or more - I'm not sure.
3. Retian the old scores from the earlier systems and create a way to make them relevant to each other.
4. A need to eliminate most of the fanboi and hate scores.
The way this was done, from what I remember, was to select a value and normalize all the scores in each period using that value as the mean for all the scores in each period. This was applied to every score given to a story. Also, when the three aspect scoring system was killed only the Appeal score was used. I think the chosen mean was 6.
So each scoring period was processed by itself to rate all the scores given in that period relative to each other, then the mean of those scores calculated. From which a value was calculated to have a mean of 6, and that value applied to all the scores to adjust them to have a mean of 6. This was done to each the scores of each scoring period as a separate calculation so each period now had a mean of 6. This allowed all the scores given to now have a valid value relative to each other, and meant they could now all be used in a single calculation. In short - this action took a bunch of mixed fruit scores and made them all apple scores on a grade curve for a fair comparison.
The next stage was to calculate the individual score for each story. That is done by removing the top and bottom 5% of the scores (but only in whole numbers, so they get rounded down to whole numbers), and an average for that story is calculated from what's left - and that's the official score.
There are two aspects of the system that do cause some temporary anomalies.
First the rounding down means that when the votes get to a certain point they do not cause another score to be dropped until the number of votes makes it another whole number. Example: 80 votes means 4 votes are dropped at each end, this continues to happen up to 99 votes, and only when 100 votes are in does the number dropped move over to 5 votes being removed at each end. This will see a sudden change bigger than expected by one vote.
The other is a vote at either end of the spectrum results in a change in the value of the votes being dropped. Example: Most of the scores are grouped in the middle or top end. there are 87 votes so the top and bottom 4 are dropped. The story has 3 votes of 1 and the next is a vote of 5. This means the 3 x 1 and the 1 x 5 are dropped. A new vote of one means the 4 x 1 are dropped and the 1 x 5 is now counted. Thus a 1 vote can put the score up by moving the cut off further down the curve. Similar anomalies can happen at both ends.
An example of a combined application to see a score drop is: 99 votes in place that includes 7 votes of 10, 4 votes of 1, and 2 votes of 6. Thus 4 x 10 and 4 x 1 are dropped. A new vote of 10 is made for the 100th vote, now 5 x 10 are dropped along with 4 x 1 and 1 x 6. The total to be averaged goes up by 10 from the new vote, but loses another 10 from the cut, so no change to the total for that, but the drop of the low end now includes a 6 so the total drops by 6 while the average is now done by 90 and not 89. Thus the official score will drop due to a vote of 10 at an important time. Say the story raw scores are 99 votes for 580 giving a score of 5.89 (since the lower decimals aren't rounded) calculated by 536 divided by 91 due to 4 scores at each end being dropped - 4 x 10 and 4 x 1 removing 44 points. The new vote of 10 makes the raw score 100 votes for 590. Now ten votes are dropped: 5 x 10, 4 x 1, and 1 x 6 removing 60 to give 530 divided by 90 to give 5.88 (since the lower decimals aren't rounded so what comes after the 2nd decimal doesn't show.) The anomalies are always short lived and usually vanish at the next vote.
The two key things to remember are the scores are there solely for the readers and they affect all the authors in the same way, so the system is fair.