I'm now up to 6 pairs of running shoes
JBBury, my guess is that because you're lightweight and quite probably have a pretty efficient running style, the wear is minimal on your trainers.
My current trainers are coming up to 500miles and they don't feel much different to my new trainers (same model), so they've obviously worn pretty well and have more life in than I'd realised!

In the past I found difference between new/old trainers to be pretty substantial!