It was simply because of different bloodlines that one possessed the seed of life while the other did not.
As a result of the enhancement from the royal bloodline, traditional knights were slightly stronger at the same rank. However, the New Knights, thanks to the integration of the Fighter System, could be bolstered by the Meditation Skill late in their development, thus compensating for the lack of life energy.
Overall, each had its strengths and weaknesses.
In the later stages, Adam planned to incorporate a portion of the mage system into this profession, transforming the New Knights into a profession capable of casting spells.
Spells were not exclusive to mages, nor were knights disallowed from using them.
In fact, a significant number of spells were highly compatible with knights. If used judiciously in battle, they could greatly enhance a knight's inherent power.