WP smush it - does it really work?

I have installed WP Smush it to optimize images. I tried with a 421x270 image 101kb and it optimized to 99.14kb (almost 2.91%).

Then I tried compressjpg.com and it turned the 101kb to 21kb.

So, does Smush it, really works?

Is there any other plugin to compress image on the fly but REAL compression?