If it works like Windows, it's normal. Files are in use.
By examining 1) the reason given by avast! for not being able to scan the files, 2) the location of the files, you can get an idea of what program they relate to. You may need to expand the column headings to see all the text.
Files that can't be scanned are just that, not an indication they are suspicious/infected, just unable to be scanned.