Microsoft did a lot of weird things like that - also with Outlook, which used to use Trident to render emails, when IE was finally getting a bit better the Office team decided to switch Outlook to use the far, far worse Word HTML rendering engine instead...
Even thinking of that makes me very happy I haven't had to worry about formatting email newsletters for more than 10 years now, brings back bad memories...
What makes it so much worse is that Tasman, the engine used in IE for Mac, had awesome support for CSS, transparent PNGs, and more years earlier, but for some reason it got dumpstered when IE for Mac was discontinued while the inferior Trident lived on in Windows.