They are Holy Knight and Divine Knight abilities, not Knight abilities. Lets look at some examples, real life and Tactics, where having something as part of a name doesn't mean it is really so.

Tactics:
Angel of Death-gee, it must be an angel. Actually, it's a Zodiac Demon.
ArchAngel-see above.

Now, lets look at real life examples.
Intel Pentium III-Pentium III does not equal Pentium. Pentium, however, is part if its name.
Microsoft Internet Explorer-Is it the internet in itself? Is it some kind of person going to a farawar and unknown land to explore?

Lets step off the really obvious and infinite examples and actually examine the classes themselves. Are Holy Knights and Divine Knights really Knights? No, Knights, Holy Knights, and Divine Knights belong to 3 distinct jobs, with no overlayering abilities whatsoever. According to the game itself, they are separate entities, not to be confused with each other.