Well the quality seems quite a bit worse for the picture I tried. Original: http://postimage.org/image/94ol50ck/, compressed: http://postimage.org/image/14712qgjo/
The resulting image has way less detail in higher frequencies.
I'd like to run it through errorlevelanalysis.com but their service seem to be down.
I suspect it tries to raise the brightness and sharpness of certain pixels before compressing it further so "important" elements stay in detail. What was done to your image though looks nothing like the quality of their examples for the ship, etc.
> The resulting image has way less detail in higher frequencies.
But the compressed version actually exposes more detail.
No. It really doesn't.
It adds a couple of visual filters (brightening, sharpening) before recompressing, but these don't "expose more detail", but rather all three steps actually introduce additional errors. Errors that trick they eye into seeing a "better" image, but errors nonetheless.
Are you attached to JPEGmini? If so, would you be willing to bet that I cannot tell if a photo of my choice has gone through your system and been compressed?