Different boundary in multipart/form-data header and request body using XMLHttpRequest

Question

I have an issue with sending formData using XMLHttpRequest. In my request header, the Content-type: multipart/form-data; boundary=-----MochXhr1234, but in the formData boundaries, it is ------WebKitFormBoundary3sEuJp62GyQo49IC. I didn't use any proxy or change headers anywhere. I also didn't set the Content-type manually.

I'm expecting the boundaries to be equal.