Well, yeah Anko cursed's seal was sealed by Sandaime, but Sasuke's cursed seal was sealed by Kakashi. We also don't know if she took the same pill as Sasuke did, but I doubt that Anko is just another experiment. I think Orochimaru let Anko go since she didn't want to stay or follow him, in the fillers arc orochimaru said that he needs someone with the desire of power and that she lacks that.
So why Kabuto tells kimimaro that Sasuke's got the cursed seal of heaven as new container for orochimaru's dream, after doing reference to Kimimaro's earth cursed seal?. I think that Anko has a hidden ability or some kind of kekkai genkai which called orochimaru attention. I just base my opinion on what Kabuto said about the seals.