Google is testing showing just a single star icon for the star rich results, even if the rich results should show multiple stars. This was spotted by Vlad Rappoport and here is the screen shot he posted on Twitter.
You can see both search result snippets have over 4 stars listed numerically in the snippet but the star icon only shows a single star. For example, Best Buy has a 4.8 star rating with 505 ratings but it shows a single star icon.
For more on the schema and markup for this rich result see the review snippets docs.
Forum discussion at Twitter.