Ok just to say, I guess you learn things all the time from observation. Here is something, Google will give different SERP results based on capital letters or not. I rank page 2 for "Keyword Keyword" but no "keyword keyword" I've bolded the letters in question here Now if you understand what I mean. Also On my site I have a <h1> tag with "Keyword Keyword" I think this might be the problem. Now I should probably edit that h1 tag and uncapitalize it sheez... People are less likely to use capitals in there search. Go on Google and try a keyword seach by capitalizing or not. Depending what your searching for the results may change slightly or entirely. Just by a Capital letter.
I've did a few tests, I see the same results. What are the keywords in question? I'll be it's related to the fact of what the keyword(s) are. Are you checking the same data center for results? Searching for the same term back to back will bounce you to a new data center within that region to prevent abuse\attacks.
Well if thats what its doing its still changing the SERP results. For example search "Cool Time" & then "cool time" I can see the #5 and lower listing changing. While the Very top listing don't change.
that's why it's always recomended to write keywords without capital letters becuase that includes the Keyword on it and that doesn't work the other way.