I work in fashion retail. We stock clothes from several different suppliers and I agree that sizes can vary quite a lot. It can depend very much on the target consumer group. A size 14 top aimed at teenagers would probably measure about the same as a size 10 top aimed at 'the more mature lady'.
And believe me, we've seen it all in our shop... some people are just so much in denial about their size. Why??? Try it on and if it fits you and you like it, buy it. Never mind what the number on the label says!!!
We have one regular customer, in particular, who is a size 20, who, when she buys something she always asks us to swap the size cube on the hanger to a 16 so that her husband thinks she's a smaller size - ?!!!
And another lady who's size 18-20 and buys everything in size14 without trying them on, then returns the lot a few days later. Whenever we manage to persuade her to try on things in her proper size she looks lovely in them, but never buys them.