Where the extra 13 bytes are coming from in Chrome DevTools > Network > Size

I’m wondering where the extra 13 bytes are coming from in “Chrome DevTools” > “Network” > “Size”

Chrome defines the size like this:

Size. The combined size of the response headers plus the response body, as delivered by the server.

Now take this file test.html:


    Test caching behavior ...

You can copy / paste the source code in an online tool, like this one: https://lingojam.com/ByteCounter

The amount of bytes is: 143

Now i do this:

Now DevTools is showing:

Size: 464 B

Now i click on the row, so i can view the source of the response headers:

HTTP/1.1 200 OK
Date: Fri, 17 Nov 2017 16:30:40 GMT
Server: Apache/2.4.17 (Unix) OpenSSL/1.0.1e-fips PHP/5.6.16
Last-Modified: Fri, 17 Nov 2017 14:51:35 GMT
Accept-Ranges: bytes
Content-Length: 143
Cache-Control: max-age=9, public
Keep-Alive: timeout=1, max=100
Connection: Keep-Alive
Content-Type: text/html

This is 308 bytes

So conclusion:

The response headers:   308 bytes
The response body:      143 bytes
--------------------------------- +
                        451 bytes

But DevTools is showing 464 bytes, instead of 451 bytes. So the difference is: 13 bytes.

Where are those bytes coming from? This example is about Chrome, but i think i saw the same behavior in Firefox.