If you've kept your front struts fresh, you've kept the car from bouncing / flexing in ways it shouldn't, which also helps prolong life of rear ones. I would guess that you've replaced front ones and kept alignment correct, tire pressure, etc. This, in my experiences, has a lot to do with rear ones staying tout, as they just don't have the same work load. Plus, if you've driven the car responsibly and have a good % of highway miles, it justifies them being OK even more so. Just my 2 cents...