Likely, Carvin specs the string from LaBella, and buys in bulk, to save a bit of money, and pass along the savings.
I bought a set of the Carvin flats for a 4003 a couple of years ago... Played, felt and sounded so good, I received an offer I couldn't refuse! So I sold it!
The only string I have played that I like better than LaBella (and/or Carvin strings) is the Pyramid Gold (Hofner strings are made by Pyramid, so they are included). They have a "medium" tension, complex overtones and a silky feel, with no break-in time, and they last for years!