I suppose it's a bit like the way 'turnip' is used in different parts of the UK.
It is and it's not, Ishbell.
With turnips we're talking about regionally different names for the same vegetable. As you point out, in the U.S. it's mostly called "rutabaga." But in some parts of the U.S., and in Europe, it's called a "Swedish Turnip." I've always thought that the British term, "Swede," was merely a contracted form of that.
With pumpkins the confusion goes deeper, because there is no such thing, horticulturally speaking, as a pumpkin. So the term---which dates from colonial days, when Europeans coming to the new world called all squashes "pompions"---is applied to a vast number of hard-skinned squashes. Most usually these are round, orange ones. But, as we've seen, that isn't always the case.
Contributing to the confusion: Very similar names for different varieties. You may have noticed, for instance, in my last reply to ChefBazookas, that there is both a Sweet Potato Pumpkin, and a Sweet Potato squash on the list. Those two actually are different species.
As a matter of historical interest, the model used for Cinderella's coach was a large French variety, which, while generally ovoid, is slightly flattened.