In the search results my client's website first sub-listing is not correctly displayed like the titles of other pages, despite using the same structure method in making the pages.
Why does this occur and how could I solve it?
I assume you're referring to http://www.andreexpress.be/diensten/ramenwasser/.
Was the page's
<title> tag ever "ramen en vitrines"?
If you changed the title after Google indexed the page, Google is probably using a cached version of the page. Give them time to re-index the page and the title will be updated accordingly.