The selected JSON body is counted before it is sent.
The selected body is exact bytes
The body display row is readable. The exact selected body bytes are represented by the hex row.
body hex is exact
The JSON body has fourteen bytes
Counting the selected body bytes gives 14.
body bytes=14
Content-Length matches
The response Content-Length row also says 14.
Content-Length=14
Summary
The response says JSON and sends the pinned JSON body length.
response length checked