JPEG Quality Issue

[d0m1n1k]

Hi gimp users,

I got JPEG images from a multifunction copier. I really like the quality per size of these images!
I would also like to use this jpeg quality with gimp exports. But this doesn't work as expected. First I opened an image generated by the copier. Then see attached 3 screenshots from export dialog:

1 - these are the standard values. It produces a very big size jpeg. This is not what I want.

2 - these settings are automatically choosed, if I just check the "Use quality setting from original image" checkbox.
The exported image with this settings is very good. The size is even a bit smaller than original. This is what I want! I like to use this quality settings for default at all images!

3 - But if I uncheck the "Use quality setting from original image" and keep all other settings changed before, I got an image with horrible quality.

Why can I not use this quality level by choosing the settings gimp choosed? It's absolutely insane: The only workaround I found to use this quality level is:
1. Open my own image (not generated by multifunction copier)
2. Copy the whole image to clipboard (Ctrl-C)
3. Open any image generated by the copier
4. Chance canvas size of this image to the image size from my own image
5. Paste my own image from clipboard (Ctrl-V)
6. Export with "Use quality setting from original image"