Difference between Safari and Webkit App ??

I am confused about this. I know that Safari uses the Webkit rendering engine. However, there is commentary on the web indicating that the most up-to-date Webkit open source application version is better than Safari. For example, while both the latest Safari version and the latest Webkit download both pass the webstandards.org Acid 2 test, there is a relatively new Acid 3 test that is passed 100% by the latest (8/31/08) nightly Webkit version, while the latest Safari version does not pass. But here is the puzzling thing: When you install the latest nightly Webkit download, and click on the program icon (which looks like the Safari icon except the compass has a gold instead of silver circumference) the program that loads looks in every say like Safari -- indeed the menu bar says it is Safari and if you look at the "About Safari [sic]" info, the version info, including version number, is exactly the same as the info (including number) that you see if you launch and check Safari itself (silver compass). In other words, the program appears to be identical except for the icon coloring, but one passes Acid 3 and the other doesn't. What's going on here?