Lia,
please don't take this the wrong way...the answer to why would anybody fake it is simply "Money".
A few years ago there were a bunch of blue rope smurfettes being sold for very high prices on ebay and there was a rush among collectors to get their hands on it which of course made certain sellers more willing to "find" more of those smurfettes and to sell them. I still see the blue rope smurfettes being sold as very rare items on ebay. Until you mentioned it, I had never seen this marking refered to as promo, so that would explain why it's not listed as such in the DSK.
I am not saying yours is a fake..and I hope it's a genuine promotional item for your sake and everybody else's sake who has one of them, but my personal opinion on them is that it's easy to create those and as Andre said, those markings are all over the cables which might explain why there are blue rope smurfettes with different markings or with no markings at all.
In the end, it will be up to each of us to decide for ourselves if the blue rope smurfette we have is a genuine promo item or not..Like with everything else, there will always be doubters and there will always be believers.