They are just struggling for any consistency, just like many shops.
BTW, I don't understand how anyone can say "I'm a 12" or whatever. Everywhere is so different. I often take a tape measure with me, as I can't be bothered to lug multiple items into the changing room, just to sort out a size, never mind style/colour.
In my mind, I AM a 12, as I have gained only a little weight over the years and haven't really changed. But that's a 1980 size 12. In reality, I have now miraculously "shrunk" to a size 8 maximum in many shops 
The up-sizing of clothing is entirely to blame. Shops know that people like the lowest possible number on things they try on. Anyone who sells clothing can tell you that some people who are convinced they are say, a 12, will categorically not buy something if it's label is too "big". As long as we buy into this delusion, they will continue to do it. It is damaging for health, IMO, as having a 33/4in waist is now "only" a size 16 for many shops, which sounds OK. To an early Eighties woman, that would be a size 20, which would give pause for thought!