With the plugin WP Minify Fix, all internal JavaScript and CSS files should be minified.
However, the summary amount of gtmetrix.com also still depends on many other factors. For minification so only the sections “Minify CSS” and “Minify JavaScript” relevant.
At these two portions, the amount is close to 100%. That this amount is often not exactly to 100%, but “only” ~ 98%, is because not all can be minified JavaScript and CSS files. The following files can not be minified:
- External JavaScript and CSS files from other websites (if it is not set in the preferences)
- JavaScript files that are inserted from another JavaScript in the page can not be minified
- JavaScript and CSS files that are loaded via PHP, and have a complex URL
Also, I have just found that the minification of gtmetrix.com the files again by 1% – 2% can reduce. This relates to the used minify engine, and not the plug-in itself. Probably the [Minify engine](https://code.google.com/p/minify/) can be improved, so you should contact the appropriate [issues page](https://code.google.com/p/minify/issues/list).