When a DOCX document contains a table with fixed layout, and the table contains <tblGrid> element with improper widths (not coinciding with the ones visualized by MS Word), and such document is imported, the table is laid out with the width of the sum of the column widths imported from the table grid (tblGrid), instead of ones that could be correctly computed by the preferred width of the table cells. 

MS Word, for example, may ignore the widths specified in the tblGrid and compute new ones.

As a side effect, fixed-width tables can be laid out longer than their preferred width.