What are this gap mean in Chrome devtools profile flame chart

Here is my javascript code, it is pretty simple:

console.profile();
var count = 1000;
var fn1 = function () {
    for (var i = 0; i < count; i++) {
        console.log("THIS IS FN1");
    }
}

var fn2 = function () {
    for (var i = 0; i < count; i++) {
        console.log("THIS IS FN2");
    }
    fn1();
}

fn2();
console.profileEnd();

and this is my profile screenshot:

Why there are some gap in the image, just like my black rectangle marked?

What does this gap mean?