The number of words in each line of ancient poetry should be equal. Why is the first line of Goose Fu three words and the last three lines four words?

Ode to Goose is an ancient poem or ballad, and there is no strict word limit. Later, modern poems developed, such as "Mountain in Autumn" by Wang Wei, require words to be restrained, two or three must be confronted, and the quadruple of * * * is divided into five words and seven words; Couplets that only write two modern poems are called quatrains, which are not necessarily antithetical sentences, such as Li Bai's "Smelling the flute in Los Angeles on a Spring Night".