Link discovery time and deep crawl rates immensely effect SEO. Just because you make changes to your pages does not ensure that those changes are acknowledged by search engine spiders.
Since your data exists in a cloud (across multiple servers) referencing vast pools of data. One change to any portion of a website (allintext ratio) impacts the global profile of the collective website as a whole.
For example, if you have clearly mapped out specific pages to rank a particular range of keywords, then you need to ensure that your SEO is fine-tuning or monitoring the impact of what happens when you add new content or change existing content within the site.
One aspect of this fine-tuning is understanding the role of global keyword density (which represents how many keywords are prominent and in what concentration your site has for each word in combination with other relative keywords).
Another aspect is the link weight and the ratio of internal links to external links for each page, the allintitle relevance (how many pages have the keyword in the title), allinanchor ratio (how many people link to the site and the page) also affect positioning.
Going back to the originating statement about discovery time, since search engine spiders crawl a site at different intervals, it is common that not all of the changes implemented are accounted for (if you publish content frequently).
The message is, have a consistent schedule for publishing new pages, for example if you blog utilize frequency (to keep the crawl rates higher). This alone can introduce spikes in traffic through RSS being published but more importantly it is ideal to deep link to other pages that are starving for needed link equity.
On page adjustments can do two things (a) improve positioning or (b) undermine trust, if a page has been the same for months, then it is altered and crawled you may in fact experience a temporary loss in relevance for the aspects that had trust and stability.
Making backups of older files and archiving them for this reason is essential if you experience a dip in rankings for your main keyword or are simply losing traction across the board for your site. Changes should be implemented slowly over time for major revisions or additions to a site.
For example if I have 100 articles I wished to introduce as white papers, adding them all at one time would have less value than spreading them out over 2-3 months at a publishing rate of 2-3 per day. Everything leave a trail and algorithms are ma thematic, so shifting the focal point so abruptly could trip a penalty or algorithmic filter.
The takeaway is, examine the crawl rates on your top ranking pages and ensure that they are getting enough attention, flow of fresh links (from within the site and from external links). Competition exists and is hard pressed to take your spot at a moments notice unless you keep constant vigil for your website and its most precious rankings.