Rationale (April, 2017):

The unification of the “in the form” pattern is confusing, so the question was based on a misunderstanding of the text.

Subclause 9.1 [dcl.pre] paragraph 1 defines simple-declaration as:

simple-declaration :
    decl-specifier-seq init-declarator-listopt ;
    attribute-specifier-seq decl-specifier-seq init-declarator-list ;
    ...

However, 9.1 [dcl.pre] paragraph 2 then refers to a simple-declaration using a different producction:

A simple-declaration or nodeclspec-function-declaration of the form

    attribute-specifier-seqopt decl-specifier-seqopt init-declarator-listopt ;

is divided into three parts...

It appears that the latter redefines the grammar non-terminal simple-declaration in an inconsistent way.