"Quality" of JPEG images

I am quite happy with IM preserving the JPEG quality by
default, i.e. 
producing a JPEG output of comparable size as long as the
dimensions of 
the picture are not altered.

But how exactly does IM identify the JPEG quality when I use
the command
identify -verbose test.jpg

? Having tested this with different image sources, I suspect
that this 
just evaluates a JPEG header field. Depending on the program
source of 
the image, the same photo can have quite deviating
"quality" though the 
JPEG files have almost the same size.

Any ideas?

Wolfgang Hugemann
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