After reading through mozilla docs I found this:

In the global execution context (outside of any function), this refers to the global object, whether in strict mode or not.

After playing with scopes for a little I found that in node.js REPL...

> this === global
true

but when I create a Script with the same line...

$ cat > script.js
console.log(this === global)
$ node script.js
false

Is there a reason for this? Or is it a bug?

Node's REPL is global. Code from a file is in a "module", which is really just a function.

Your code file turns into something like this very simplified example:

var ctx = {};
(function(exports) {
    // your code
    console.log(this === global);
}).call(ctx, ctx);

Notice that it's executed using .call(), and the this value is set to a pre-defined object.