How do earth do Amazon rank their customer reviews?

I notice that by default Amazon sort customer reviews by "top-rated". Even when changing to "order by date", it reverts back again when you click on "see all x reviews". If there's a fair few reviews this is important since any potential purchaser will likely only read the "top rated reviews". But I'm thoroughly mystified as to how they are determining their ratings. Let's take an example of a review that I submitted for a pack of six BIONA Organic Vegetable Juice cartons. I'll paste my review in here for convenience. 

I can't imagine people would drink this because they actually like the taste. Presumably they'll buy it for the health benefits? It's easy enough to drink. Not quite as drinkable as V8 veg juice, but the V8 is 87% tomato juice! This seems less than 87% tomato juice and hence there will be a greater percentage of other vegetables. It still tastes predominantly of tomato juice though (it doesn't tell you the respective percentages, why on earth are they not legally obliged to give this information?).

Here's what it contains -- Tomato Juice, Carrot Juice, Beetroot Juice, Cucumber Juice, Celery Juice, Sauerkraut Juice, Onion Juice, Sea Salt, Lemon Juice, Spice Extracts, Dill Juice.

All "certified organic juices".

Onion juice?? . . . .but anyway. The sodium content is a concern at 0.16g per 100ml. This is only 2/3rds of the sodium content of V8 Veg juice. However it is still a HUGE amount of sodium -- I believe that anything over 0.6g per 100g/100ml is defined as a high salt/sodium food in the UK (to get the amount of salt you multiply the sodium figure by 2.5). And it is recommended that you consume no more than 2.4g of sodium a day. One of these 500ml cartons will contain 0.8g of sodium, so if you drink a carton a day you'll be using up a third of your recommended maximum sodium/salt intake! 

I've been drinking a quarter litre a day of this juice for the past 2 weeks. Not sure if it's having any benefits, but I'll continue buying it and drinking it. 

In conclusion it seems it might be a better choice than the V8 alternative, although it is quite expensive. I have to order it from Amazon as it doesn't seem to be sold in any supermarkets where I live.

There are at this moment 25 reviews altogether.  Mine is rated by Amazon as the 23rd top-rated review -- or in other words Amazon rate my review as being the third worst out of the 25 reviews.

My review received 18 helpful votes from the public.  No other review gets more than 2 helpful votes and most reviews don't get any helpful votes at all.  Moreover, my review contains vastly more relevant information than any other review.  One of the reviews, the 7th top-rated, merely says, "first time, lets see if any good". This, of course, isn't a review at all, yet Amazon rate it as one of the better reviews. So I'm mystified as to how Amazon are rating customer reviews. They're not going by their customers opinions. They're not going by how useful the review is to a potential purchaser. So how on earth are they determining the ratings? Any answers below are very much welcomed!