Google+ Local scores - How Google Determine Average Score

Google+ Local scores

All scores in Google+ Local are determined by user reviews.
Individual user ratings are based on a 0 to 3 point scale.

3	Excellent
2	Very Good
1	Good
0	Poor to Fair

We take these ratings, average them, and then multiply by ten to arrive at averaged scores.

26-30	Extraordinary to perfection
21-25	Very good to excellent
16-20	Good to very good
11-15	Fair to good
0-10	Poor to fair

When you’re searching for locations in Google+ Local, you may see these scores depicted in two ways:
Scores with multiple aspects
These scores will show you several scores for different aspects of the business. For example, restaurants are scored across three areas--food, decor, and service--with food being the primary aspect.
Overall scores
When we don’t have enough user ratings on different aspects, we will just show an overall score. An overall score is comparable to a score in the primary aspect for a location, like food for restaurants.

Resources https://support.google.com/local/bin/answer.py?hl=en&answer=1723748&p=zagat_score