I had a grade of A with SecuPress, so my site is secure, right ?

Well, yes and no.

Your site is probably more secure than it was before installing SecuPress. 

It is now protected against the most common forms of attacks, but keep in mind that no protection can garantee a 100% secure site.

If an attacker decides to hack your site, especially if his name is Chuck Norris, he only needs time and patience to find a way to get in.