JSON data that starts with closing brackets

Question

I found a JSON file beginning with closing bracket is used in one of google's services. The following is an example, you see it has closing brackets and a single quote before the main data part. I inspected javascript code where this JSON data is fetched. And what they were doing was to remove the first sequential brackets manually before parsing the data.

)]}'
["value1", "value2"]

Here're my questions.

1. Is this JSON format valid?

2. Anybody have idea why they're doing this?