Why is since_id not in next_results link included?



I´m currently programming a script, that´s searching tweets over REST API v1.1 SEARCH request including a special hashtag.
After that, I store all Tweets local to retweet relevant tweets from my results.

My script is checking the rate limits and caches it local to compare it after each new request. So, some results of my search return more than just one page of tweets! That’s why I like to treat it with care and use since_id to request just tweets I did not process on my server already. Okay, so far that’s working fine for the first result page.

On the end of each page is the search_meta called next_results, what delivers a set of parameters to request the next page.
Now I’m asking you, why is the previous used since_id not included in the next_results parameters.

If you use next_results as the API delivers it, you get all tweets found until your count limit for that page and several more pages far beyond the previously defined since_id. Is it normal that I have to add since_it again to the parameters of next_result before setting the request?
Or is there any better solution, to get all tweets, even with multiple pages, but only to a specified ID with since_id?