grammaticality - The reasons are two. Correct?

I stumbled upon this phrase in this video's description.

A few words about this masterpiece: Brahms began composing this piece in 1854 and finished it in 1876. The reasons for delaying finishing it are probably two. First, Brahms' self-critical fastidiousness, and secondly, the expectation from Brahms' friends and the public that he would continue "Beethoven's inheritance" and produce a symphony of commensurate dignity and intellectual scope, an expectation that Brahms felt he could not fulfill easily in view of the monumental reputation of Beethoven.

Is this grammatically correct? Can a numeral be a predicative in plural stating the number of the objects that are denoted by the grammatical subject? Could one say, for example

The pens are two, not three.

instead of

There are two pens, not three.

If the sentence from the video description is correct English, while my example is not, please explain the difference.

Answer

The original sentence is grammatically correct, as is yours. There are several possibilities as to the why of the structural choice.

In highly conjugated and declined languages, such as Latin, a word's grammatical role is usually clear based on its form, and placement is used for emphasis. Placing words first or last in a sentence, regardless of whether it is the subject, verb or object, tends to indicate its importance to the idea.

Because English is not highly conjugated or declined, placement is often the principal indicator of a term's grammatical use in the sentence. The following sentences do not mean the same thing:

John hit Peter.

Peter hit John

However, there are structures in English that allow reorganization of position without making the grammatical meaning ambiguous. In those cases, position can be used for emphasis. The predicate nominative is one of those types of structures.

In the following sentence, there does not seem to be any significant emphasis on any particular word or term.

There are probably two reasons for delaying finishing it.

The original sentence has a somewhat less common structure. By itself that creates a bit of emphasis on the whole sentence.

The reasons for delaying finishing it are probably two.

Additionally, placing reasons at the beginning of the sentence seems to place a slight emphasis on the word, both because of its initial placement, and because it is in a location where it usually would not be found. If it were spoken, one can imagine a rising inflection on the first syllable of reasons.

Further, alternative structures are often used just to make a sentence seem a bit less mundane and to avoid repetitive, sing-song pacing.