They wanted a way to get the type of declaration of an identifier.
They also wanted a way to get the type of an expression, including information about if it is a temporary or not.
decltype(x) gives the declared type of the identifier x. If you pass decltype something that is not an identifier, it determines the type, then appends & for lvalues, && for xvalues, and nothing for prvalues.
Conceptually you can think of it as the difference between the type of a variable and the type of an expression. But that is not quite how the standard describes it.
They could have used two different keywords to mean these two things. They did not.