Correction: Non-breaking spaces (i.e. ASCII 160) in Word 2013 & later have a variable width; in Word 2010 & earlier they have a variable width. Your document's justification will still work, but the results will vary according to which Word version you're using and, if it's Word 2013 & later, whether the document was created in an earlier version or with compatibility set for Word 2010 & earlier.
If the document:
• is opened in Word 2010 & earlier, the non-breaking spaces will have a fixed width regardless of the version the document was created in;
• is opened in Word 2013 & later and the document was created in an earlier version or with compatibility set for Word 2010 & earlier, the non-breaking spaces will have a fixed width; and
• is opened in Word 2013 & later and the document was created in Word 2013 & later using the Word 2013 & later defaults, the non-breaking spaces will have a variable width.