XAMPP VS IIS answer to Incorrect $Ajax

I have a collegue who uses XAMPP(5.6.3) to develop and i am using the IIS(7.5).

I have had a scenario today where The IIS answered to a wrong(at least i think its wrong, I am relatively new to web development ;) ) ajax request with a "405 method not allowed" but the XAMPP accepted the request and returned a JSON.

The ajax call looks like this:

$.ajax({
    async: false,  
    type: "Post",
    url: "./subdir/subdir/mydata.json",
    success: function( data ) {
       //some code here
    }
});

Is there a reason for it ? Is there a RFC which states what the correct answer should be ?