traits2.rs Function Equivalence

[Savaesthetic]

In the traits2.rs solution, the function signature for append_bar differs between the trait definition and the Vec<String> implementation.

trait AppendBar {
    fn append_bar(self) -> Self;
}

impl AppendBar for Vec<String> {
    fn append_bar(mut self) -> Self {
        //        ^^^ this is important
        self.push(String::from("Bar"));
        self
    }
}

Why does this compile if the function signatures differ?

My understanding is that because the function is not taking a reference, that the mut keyword is stating that self is mutable within the function. This seems to offer an additional level of freedom compared to the trait signature which states that self is NOT mutable.

In this case how can these two function be considered equivalent?