Microsoft Ends IE for Mac
Actually, I suspect that the idiosyncratic similarities are more along the lines of IE5 Mac attempting to respond similarly to IE6 Win for quirks mode. Strict mode, on the other hand, is something else entirely, and IE5 Mac is in many ways a far superior CSS2 implementation than IE6 Win. If anything, IE5 Mac follows CSS2 more closely than just about any other browser - Firefox and Safari's implementations are much closer to CSS 2.1. This leads to some fairly unique behaviour on IE5 Mac's part where some of the more erroneous parts of CSS2 have been implemented quite strictly (width of floats springs to mind as an example).
Drawing a direct comparison between IE5 Mac and IE6 Win is therefore just not a correct thing to do - they are really quite different browsers in many ways, and testing a site in IE5 Mac is certainly no substitute for testing in IE6 Win in any circumstance.