Finished Cowichan Sweater

I finally finished the Cowichan Sweater that I've been working on for a year. I designed the Inukshuk myself as I was knitting this during the 2010 Winter Olympic Games and thought it would be a nice touch. I also added the Maple Leafs and the "Canada" along the bottom instead of the X's the pattern called for. Sewing the zipper in was a huge pain in the A$$ and I hated every minute of it. But, I finally got it done, gave it to D, he put it on and then I noticed the GLARING error!!!! What happened there? Not sure why it doesn't match left to right. All sections measure the same. I told him of the mistake and said I would need the sweater back to fix something but he said it was the warmest sweater he had and didn't want to give it up. He's growing so fast it probably won't fit him in a few years but until then, he is perfectly willing to live with my mistake! I don't even know what I did wrong so I don't repeat it. Any ideas?