How is ImageOptim a better image compression program than Photoshop? What are Photoshop users misunderstanding with regards to true image optimization/compression? Thanks!
Photoshop isn’t the only option 
I think PS’s optimisation tools are still lacking, I regularly get images that have passed through PS’s “Save for web” feature and often still manage to shave an extra 10/20/30% through my own image editor.
Adobe was very late to the web, it didn’t even support the .jpg format without a plugin when other images editors were leading the way (read http://royal.pingdom.com/2009/10/20/adobe’s-bloodless-coup/ )
Do you know what the difference is? That is, what is Photoshop not doing?
I don’t know the technical differences, only the developers would be able to tell you that - it could be that ImageOptim is using more up to day methods and technologies than Adobe.