Sorry it wasn't clear. The blood test before the superpower selection shows which powers they can potentially have and once a superpower has been picked then they are injected with a catalyst in order to develop that power in a stable way. The first time around the main character chose mental manipulation as her superpower which meant when she was tested again a year later there were no other possible powers as she had already received one catalyst and so her latent powers were unable to develop in any other way.