I find clothes sizes utterly frustrating too. Yesterday I bought some Nike track pants, fitted well, (labelled Medium, Nike label said 12- which I aim for one day!!), then in Tesco I bought a couple of T-shirts for work, which are a snug 16!!!
i have clothes that I wear that are in a huge array of differing size labels, its ridiculous. I have taken to picking an item up, if it looks about right then I will try it on. I try not to set too much store by labels now, I try to go by how I feel about my own size and then buy clothes that fit! (If that makes sense!)
Mind you, when I was a 22/24, and then finally was able to buy 12s (back there soon I hope!) the size was all important!! How daft am I??