Find & replace in visual studio not replacing all occurrences

I am using ctrl+shift+f to trigger "Find and replace", going to the tab "Replace in files".

I do a regexp search in the entire solution (which contains around 11000 files) for:

#layout(\d+)-top

replacing that with

#layout$1 .layout-top

A lot of files are changed by the operation, but when I after the operation open a certain css file, I still can read lines like:

#layout5-top,
#layout6-top
{
    width: 960px;
    height: 104px;
    margin: 0 auto;
}

Why?

If I do the same search & replace with that file open, the operation will affect the file.

I can find numerous files that are not changed by the way.

In the progress indicator that comes up when the search and replace runs, I see that every file is processed, not just open files.