Thanks @IgorBrigadir. Yup the 100 tweets is just the first page, but I don't think they are using 50 million tweets per day x 7 (Search API extracts tweets for the past 7 days) as the base to do the query request.
For optimization: I am interested to know what does Twitter means by optimization, what's the logic behind. How to calculate the relevance - base on no of favorite counts or other machine learning algorithm?
For the 100 returned tweets, I wonder if there is max limit for the "more page". (but due to the rules setup by twitter, every extra page is count for 1 request, and there is a rate limit) But it seems that if i made the request within a short timeframe, the returned 100 tweets with different results. May be i should made a simultaneous request to understand more the logic. However, I am thinking if there is randomness and extra logic built in.