Why if let with reference to some yields reference to inner value?

Question

The following code doesn't compile because assigning a to b returns: expected struct std::string::String, found &std::string::String. I thought &s was &Option and not &Option<&T>. It seems that &s is behaving exactly like s.as_ref(). What's the reason behind this behavior?

fn main() {
    let s = Some("string".to_string());
    if let Some(a) = &s {
        let b: String = a;
        println!("{}", b);
    }
}