Where exactly do Keyword Sandbox Additional Keywords come from?

[Just Guessing]

When the Sandbox suggests "additional keywords for you to consider", it says

Users who searched for your keyword(s) also searched for the following terms.

Does that mean:

1. Users used them in combination with your keyword?
2. Users used them in other searches?
3. Both? (my guess)

[goodroi]

I would assume both also. But I don't think it matters. It is valid data that helps with adwords and LSI.

[Just Guessing]

I'm wondering how valid the data is. If they come from other searches, is it the search the user did immediately before or after their search using your keyword? - How is Google sure that the other searches are related to the search term in question?

I know we can (and do) make our own judgement as to whether the keywords are relevant, I'm just interested in why Google thinks they are relevant.