Word changed the justification algorithm with the introduction of Word 2013. Presumably, saving it with compatibility for Word 2010 or earlier will default to the older algorithm.
See:
Document.SaveAs2 method (Word) | Microsoft Learn and
WdCompatibilityMode enumeration (Word) | Microsoft Learn
As for other justification options, see:
WdParagraphAlignment enumeration (Word) | Microsoft Learn. I haven't tried the JustifyHi/Low/Med options, so I can't tell you what they do.