I don't have Safari to test, but it looks like it does two things differently to other browsers. The first has been mentioned — slurping data from the local address book and feeding it to any site.
The second I can't test, but looking at the screenshot, it appears that Safari actually fills in the text field with the contents of the auto-fill, before you actually choose that auto-fill, and it highlights that text so typing something overwrites it, which is a standard way of doing autofill. However, using script to ask the textbox for its contents then exposes sensitive data. In contrast, other browsers display the sensitive data in (inaccessible to script) browser chrome, so no data is exposed.
Safari: R|*obin Message* (in the textbox itself)
Firefox: R|
Robin Message (in a popup, push down to get to it)
The second I can't test, but looking at the screenshot, it appears that Safari actually fills in the text field with the contents of the auto-fill, before you actually choose that auto-fill, and it highlights that text so typing something overwrites it, which is a standard way of doing autofill. However, using script to ask the textbox for its contents then exposes sensitive data. In contrast, other browsers display the sensitive data in (inaccessible to script) browser chrome, so no data is exposed.