Hi Bruno,
Google does reserve the right to edit how page titles are displayed in search results and they are doing this more and more. In theory, they do this to actually improve user experience but because this is algorithmic, they can sometimes get it wrong or it may not look exactly how you want.
Unfortunately, there isn't much you can do to stop Google from changing the title tags. I think they are less likely to change them if you have one that is well-written and relevant, but in reality, you're not going to make it relevant for every single keyword search that your site displays for.
Here is a relevant piece of info from these Google guidelines:
"If we’ve detected that a particular result has one of the above issues with its title, we may try to generate an improved title from anchors, on-page text, or other sources. However, sometimes even pages with well-formulated, concise, descriptive titles will end up with different titles in our search results to better indicate their relevance to the query. There’s a simple reason for this: the title tag as specified by a webmaster is limited to being static, fixed regardless of the query. Once we know the user’s query, we can often find alternative text from a page that better explains why that result is relevant. Using this alternative text as a title helps the user, and it also can help your site. Users are scanning for their query terms or other signs of relevance in the results, and a title that is tailored for the query can increase the chances that they will click through."
Here are some good resources showing other examples of changing titles as they wish:
http://www.blindfiveyearold.com/url-titles
http://searchengineland.com/google-title-wrong-157819
I hope that helps!
Paddy