Why transferred bytes are bigger than size bytes in firefox dev tools?

Question

I am measuring the performance from a website. When looking at the firefox-developer-tools, I noticed a weird behavior. There is a specific JS file which the transferred size is

Answer 1

As stated in the most upvoted comment on the question:

This appears to be a rare bug in Firefox that has been fixed in Firefox 52:

FWIW this bug has been fixed and the fix will appear in Firefox 52.

– Tom Tromey Feb 14 '17 at 14:59