How does Google decide when to bold synonyms for query keywords

Question

when you type keywords in the search, Google will bold the requested keywords as well as their synonyms. For example if you type Japanese cars, the synonyms like vehicles will be bold as well. But not all of them are bold, sometimes even car is not bold either.

Is there a certain rule when Google decide when to bold certain keywords and their synonyms? or is it random?

Answer 1

I see more synonyms being bolded on page two but that's probably different for everyone. If there is a rule or algorithm like any other which Google has, they won't give a specific answer.

Two good pages about how they use these are below

Matt Cutts blog More info about synonyms at Google

and

Google Blog Helping computers understand language