The IE Team explains why:
IE8 passes the official ACID2 test hosted on http://www.webstandards.org/files/acid2/test.html. There are also a number of copies of this test around the net. One popular copy that I’ve seen of late is http://acid2.acidtests.org/. IE8 fails the copies of ACID2 due to the cross domain security checks IE performs for ActiveX controls.