Natural evolution is when you are just so awesome that you ascend to a higher state of being.
Artificial evolution is when you are infected with the blood of many many monsters at the same time.
As your body fights back, you have a chance of dying anda chance of forcing you to evolve prematurely, but only once.
Xanthe's artificial evolution inducing injections are literally just a mixture of the blood of many monsters. carefully matched together for compatibility and minimizing the odds of death.
While fighting a Chimera of the Rebis' bloodline your wounds were filled with the chimera's blood, which was actually a mixture of thousands of different monster blood that monster deryl he was synthesizing into his ultimate monster body.
This infected you, and your body managed to fight it off via a premature "artificial" evolution on you despite not coming from a syringe.