The metafile is a drawing recipe, not a finished page
WMF stores drawing operations that an application interprets to paint shapes, lines, and text. PDF packages the resulting view into pages. Some drawing instructions may be translated while unsupported effects or embedded objects can look different.
Keep the original file if future editing depends on its application context. PDF is convenient for reviewing a completed diagram, but it may not preserve the original grouping or drawing commands needed for detailed changes.