It’s far too artistic and subjective for games to be judged like that 

Why?! Judging games is always subjective...still, a professional site should always try to stay as objective as possible, that’s what journalism is about. That’s why I prefer sites who let several people from their staff test the games. Might as well get into detail then.

This about covers it

Depends on the reviewer/website.

Personally on my scale when I used to write reviews here, a 5 or so would be average. Most sites don't seem to have a large scale though. Game is god tier, good, or shit.

The scale used isn’t very important, as long as it’s used consistently.

Especially when metacrtitc jumbles them all together (with arbitrary weighting’s thrown in for good measure) and the industry actively utilises the end result.

It really helps when numbers actually have meaning behind them - as I mentioned before, AdventureGamers uses 5 star system (with half-steps) and every review has link to their scoring system:

5 stars - Classic
An instant, hall of fame classic. Although not "perfect", we award our top score only to those games that set the highest standard for quality.

4.5 stars - Excellent
A superb game that excels in just about every area, held back only by one or two notable flaws or a collection of smaller ones that prevent the game from earning full marks.

4 stars - Very good
A game of very high quality. Although some aspects might have been executed better, we would recommend this game without hesitation.

3.5 stars - Good
A solid adventure that is generally enjoyable, though it lacks enough polish or ambition to recommend without caution.

3 stars - Decent
A qualified success; the positive aspects still outnumber the negative, but the weaknesses noticeably hinder the experience.

2.5 stars - Uneven
A near-equal balance of good and bad that can make a game either fall disappointingly short of its evident potential or be mildly entertaining despite its many failings.

2 stars - Underwhelming
There are some good elements that shine through occasionally, but generally the game is not a positive experience.

1.5 stars - Poor
Shows just enough promise to hold your attention, but is buried under too many problems to be enjoyable.

1 star - Awful
Offers a tiny glimpse of what might have been, but manages to execute just about everything wrong.

0.5 stars - Unplayable
A complete waste of time with no redeeming qualities whatsoever. We would not recommend this game to anyone, at any price. Avoid.

Unrated
Not a reflection of quality. We generally do not assign star ratings to freeware releases or any commercial game that is either part of an unfinished episodic series or falls too far outside our genre standards for what defines an adventure.

I prefer a more general 5 point scale, where 3 would be "average" and people don't have this weird grade school interpretation of the score. An 'average' game is forgettable but fun, probably doomed to populate lists of 'underrated / hidden gems' on forums like these.

But for a 10 point scale? I think 5 & 6 would both be average, one slightly negative and one slightly positive.

I feel that average games should be a 6.5

A 5/10 is pretty bad in my standards, while 7/10 is 'good but flawed'

I think you are technically correct here.  But, in reality, "average" is a word used to sum up ones opinion, at least in the USA.  So, while it may be a perversion of the definition of the word, it is nevertheless a common usage of it.  

I’m pretty much the same with you, I gave it a 6 in the poll- I consider 7 as a good game- not the best but better than average 

watch dogs in my opinion was a 7- a good game indeed- good concept, could have been better with some more work and love.