Can't subtract one var from another in js

Question

I have a script which I'm modifying, it resizes an image, adds a black border then adds white borders to replicate the look of a 10x8 inch print.
This line is giving me trouble and I can't seee why!
     docRef.resizeCanvas ( docRef.width + (portraitPrintW - docRef.width), docRef.height, AnchorPosition.MIDDLECENTER);
I've already set
     var docRef = activeDocument; var portraitPrintW = 576;
The script has resized the image to 72ppi and to fit 676ppi long side. Its then added a 2px black border all around. At this point the image should be 454px wide. I want to add a white border to the left and right to give a total width of 576. So the above line should be 454 + (576-454) which is 454+126=576 therefore I'm doing a Resize Canvas 576, 680 (the current height) middle centre (with a white background - set as a var earlier).
What actually happens is the canvas is resized to be 334 wide (height is correct) - how did that happen?
Replacing a couple or vars with real number allows the script to work as expected
     docRef.resizeCanvas ( (docRef.width + (576 - 450) ), docRef.height, AnchorPosition.MIDDLECENTER);
and
     docRef.resizeCanvas ( (docRef.width + (portraitPrintW - 450) ), docRef.height, AnchorPosition.MIDDLECENTER);
also works as expected
Maybe I can't use the same var twice in a calculation, no because even this doesn't work
     var currentWidth = docRef.width;
     docRef.resizeCanvas ( (docRef.width + (portraitPrintW - currentWidth) ), docRef.height, AnchorPosition.MIDDLECENTER);
So why does ( docRef.width + (portraitPrintW - docRef.width) return 334, not 676?
Thanks for any clues.

Answer 1

I think I'm already setting the ruler units to pixels, here I'm saving them to a var in order to reset them after the scrip is finished.
        var strtRulerUnits = preferences.rulerUnits;
        var strtTypeUnits = preferences.typeUnits;
        preferences.rulerUnits = Units.PIXELS;
        preferences.typeUnits = TypeUnits.PIXELS;
and that appears to be working. The calculation returns the expected answer when using numeric values in place of some vars, as I say:
     docRef.resizeCanvas ( docRef.width + (portraitPrintW - docRef.width), docRef.height, AnchorPosition.MIDDLECENTER);
returns a different answer from
      docRef.resizeCanvas ( (docRef.width + (576 - 450) ), docRef.height, AnchorPosition.MIDDLECENTER);
even though var portraitPrintW is set to 576 and docRef.width (should) be 450.
Thanks again.