[plt-scheme] mutable lists/dicts?

>>> (dict? (mlist (mlist 1 2) (mlist 1 2)))
>> #f
>  ^^^
> 
> Mutable lists are not dictionaries.

well, yes, I understood that what's #f tell me here. 

But it doesn't really match the documentation on dictionaries (3.15). It's just said there that lists are dictionaries, and that then you have a predicate to test wether a dictionary is mutable. The question is, why aren't mutable lists dictionaries, and even more precisely, mutable dictionaries?

Even if I don't get the reason why, it may be good to state it in the doc.

-- Éric