Google displays those based on popularity and search habits. Scrolling down at the end of the search results you'll see all 8 Related searches that Google found.
Now, Google shows those at the top only if there are more than 8 related searches, but since this feature is yet not final there are still cases when the related searches are not shown at the top even if at the bottom there are 8 related searches shown - the case of searching for the keyword games.
When I say the feature is not yet ready I based the assumption on:
- the display of the 4 most popular related searches at the top of the search results is not final, because Google also tests if this would affect possitivelly user's search experience (see the first screenshot I took, you can see that the related searches are placed right under the ad, so the user might be confused if that's part of the ad or not)
- the related searches need some filtering, even if SafeSearch is off. An example is the fact that when you do a search for buy, you'll see some surprising related searches, such as buy guns and buy steroids
Whether this will remain as is or changes will be made, remains to be seen.
