I've decided to optimize a page of my website for certain keywords and I expected to see changes after the page have been cached but I was somewhat baffled by the fact that for certain queries Google prefers to display the old copy of the page whereas other queries come up with the new one.
The way I figured that out is it displayed content in the snippet which no longer was present on the page. The title was always new regardless of whether the snippet of the body of the page was the new or old one.
I understand I might miss out on how search engine works, so I would really love to have some in-depth scientific work on the subject.
It could be that some queries hit datacentres that still have the old cache of your page.
I have also observed that Google reacts much quicker on the page title change than on the content changes. I.e. if I change page title, Google will pick it up fairly quickly, but if I look at "cached" link in SERPs it shows the old content, or if I search in quotes for a piece of unique new text inserted, it will not be found although different query shows the new page title (but both changes, title and content, were done the same day).
Wait a bit longer for Google to fully pick up your changes.