You probably noticed that Google's search services show different suggestions. For example, when you type "us" in Google Image Search, the first suggestion is "us map". Typing "us" in Google Web Search doesn't return the same suggestion: "us bank" is more popular. It makes sense to restrict the suggestions to queries that are popular in each specialized search engine because they're actually relevant.
Google Book Search seems to be the only Google service that shows irrelevant suggestions. When you type "shak", the top suggestions are "shakira" and "shake weight", even if "shakespeare" would be much more appropriate. Obviously, Google uses suggestions from the web search engine and "Shakira" is a more popular query than "Shakespeare".
"Suit the action to the word, the word to the action." (Hamlet)
4 hours ago