This is likely a small bug in the emulation of IE8's Standards Mode in later browsers. Clearly, these modern versions of IE cannot replicate the behavior of their predecessors perfectly (but they do to sufficient degrees the vast majority of the time).
Using http://browserstack.com I was able to test in an actual IE8 instance. As you can see from the image below, the results were good, and IE8 (actual instance) doesn't have any issue.