A webmaster from DDTH.com (a leading Vietnamese IT forum) shows 2 screenshots about different results of the same Vietnamese keywords: 1 Sentence case and the other lower case.

The keywords checked in Google Vietnam are “Thiết kế cảnh quan” and “thiết kế cảnh quan” (landscape design).

In the first case, his site ranks 1. But the other not in top 3.

I double check in Google Vietnam, finding the same results though.

To clarify his doubts, I make a quick research and finally get the clues (posted under nick Tapher).

Google Web Search Help Guide clearly states:

Google searches are NOT case sensitive. All letters, regardless of how you type them, will be understood as lower case. For example, searches for george washington, George Washington, and gEoRgE wAsHiNgToN will all return the same results.

Not satisfied enough, I continue to Gogole and find a useful article of Barry Schwartz (editor of Search Engine Land) that I certainly missed it.

Barry Schwartz (also known as Rusty Brick, founder of SERoundTable) used to report that he saw different results with Upper case and lower case of the same keywords.

Barry then received feedbacks from Googler JohnMu as followed:

At the moment, as far as I know, we do not look at case sensitivity in web search. However, at some time in the future, we may decide to take it into account should we find that it is relevant and improves the quality of our results.

If you see differences at the moment, it is likely due to normal fluctuations and perhaps differences across datacenters. If you find drastic differences, it would be great if you could post them here or better, in the Webmaster Help groups so that we can investigate them appropriately.

The bold text above is the final clue I can have: different datacenters make the differences.