I agree that you should have ended up with two lines on the next page instead of three.
I think your questions are valid but I don't have any inside knowledge that would be of assistance. I have learnt to shrug off 'minor' pagination issues like this since changing printers results in variations on pagination. If you want exact and fully predictable pagination, use a page layout program instead of Word.
Sticking with Word, you could try a few different document formats to see if one gives you a better result. You can save the file as doc to go back to a Word 2003 format. Or try some different docx compatibility formats via code eg
Code:
ActiveDocument.SetCompatibilityMode wdWord2010