[glandium]

What is confusing is that the article introduces both derivatives (of a function and of a number) as D, when both are definitely not the same thing.

EDIT: that is, the fact that they have the same name and the same symbol is what's confusing.

[tel]

For better or worse that's very common. A big part of learning to read advanced math is keeping track of the "kind of thing" each variable is meant to be. If you do that, then overloading D is a convenience which illustrates similarities.

But if you're not a professional mathematician then such overloading is terrifically confusing. Actually, I'm fairly certain it still is even for professional mathematicians.