webbrowser.get why does the path string contain %?

Question

I have this code:

import webbrowser


def main():
    url = 'https://stackoverflow.com/users/7318120/d-l'
    path_to_chrome = 'C:\\Program Files (x86)\\Google\\Chrome\\Application\\chrome.exe'  # fails
    path_to_chrome = 'C:/Program Files (x86)/Google/Chrome/Application/chrome.exe %s'  # works
    print(path_to_chrome)
    webbrowser.get(path_to_chrome).open(url)



if __name__=='__main__':
    ''' This is executed when run from the command line '''
    main()

I usually use \\ for windows file paths and this has never failed before.

So my question is, why does the second path_to_chrome which contains the % part in the string variable work and not the first (which is actually the path to the executable)?