Not sure how I own this set..... Been bugging me for a few years

Perhaps its part of the "Complete your set" deal.Where you own enough of the pieces to get the rest of the set discounted(or sometimes even free!)

https://store.thesims3.com/completeyourset.html

Perhaps it came with an expansion that you might had brought on Origin as they had some that included a store set as a bonus since the CYS isn't available with that set.

I once had this happen with the chocolate fountain. I was in my game playing Lucky Palms as I had jusr gotten the world and noticed the chocolate fountain was available for me to use. The thing is, I never brought the fountain and it was never in my purchase history as being owned. So even though it was in my game, I went and spent the simpoints for it so that it does show in my purchase history.

Edit to add this link from Carl's Guide that has a list of EP's that had added some store content to was to be sold for a limited time. The set you have came with the Ambitions EP if you had brought it back in 2014 before the promotion was over with... :)
http://www.carls-sims-4-guide.com/forum/index.php?topic=19796.0

"Springfairy556;c-16815912" wrote:
Perhaps its part of the "Complete your set" deal.Where you own enough of the pieces to get the rest of the set discounted(or sometimes even free!)

https://store.thesims3.com/completeyourset.html

Someone in a simming group I'm a member of suggested this too.
Edit:
I looked, but it doesn't have the CYS progress bar like other sets would.

I take back what I said, I just went to my store account and mysteriously items are showing in my account as well, apparently I've had them since 2014, but I never seen them before nor were they installed...