The Word output is meticulously aligned to the GB/T 1.1 specification, which is highly prescriptive on the positioning of elements on the page. This means that the Word output uses frames and VML extensively, as the best mechanism Word HTML has to ensure precise positioning of elements. However, the use of frames makes Word documents more cumbersome to edit; the bulk of document editing should be happening in Asciidoctor, with Word treated as a write-only output format.
The use of VML and frames is mostly confined to the cover page, whose format is the most heavily prescribed by GB/T 1.1. However, Word as of 2016 suppresses space before a paragraph after a page break (though not a section break)--which means that the Foreword, Introduction, Document Title, Annex and Bibliography titles would all either lose their mandated initial space in Word, or else would all have to be treated as separate sections. For that reason, those headings are instead treated by Metanorma-GB as frames (in-line with their following text), which preserve their initial spacing.
Read about Metanorma-GB’s compliance with GB/T 1.1.