Historically speaking, yes, but perhaps intentional in the present day. Who's to say, and better yet, who's going to spend scarce developer resources on proving it one way or the other? Unless you're doing corporate, that is.
who's going to spend scarce developer resources on proving it
This was the argument many had a decade ago about IE6. Lots of people designed for IE6 first and didn't spend time on alternative browsers. They were lucky that IE6 continued to be popular for so many years, but in the end it has caused a lot of problems. These days the markets move a lot quicker, so failing to support IE9 today may prove an expensive decision in a year or two.
So you're saying its just a fluke that IE9+ supports the same standards as Firefox and Chrome? Amazing luck that.