image compression - How does optimizer software reduce file size without reducing resolution?

There are numerous applications for reducing filesize. Most of them reduce resolution (e.g. from 300dpi to 72dpi) or reduce height and width. I tried http://www.imageoptimizer.net/

Surprisingly, when uploading a small file (optimized for web) with 72dpi resolution, it will produce an image of the same size and 92dpi resolution, but the file size has been reduced to 50%. How does it work?

It should not be simple compression, as compression (which is also accompanied by quality loss) reduces the file size by 10%.