You didn't mention what "numbers" you're using, so I'm going to assume that you meant "Visits".
You first have to understand how AA attributes traffic to search engines/referrals and Marketing Channels.
Referrals, which Search Engines fall under, are set with the first hit of a browser session. Note: this is not an AA Visit! So as long as the user keeps his browser tab/window open with your website, the referral/search engine will never change, mainly because the "document.referrer" string for that browser tab/window hasn't changed as long as it remains open.
Marketing Channels are determined by your Marketing Channel processing rules. You'll need to refer to these rules in your AA report suite to determine the logic for how your "Natural Search" channel is being set.