We have no real answer to this, but one can make assumptions.
One might at first assume that Ralts and its evolution line would be sorted into purely the "Human-Like" Egg Group (Gardevoir is, in fact, based on a ballet dancer and a knight). While the line is in this group, it is also in Amorphous, which you seem to be confused at. It is also stated on Bulbapedia that Gardevoir (and thereby its evolutionary line) is based on the concept of empathy, which is abstract, ergo amorphous.
In addition, Game Freak often sorts Pokemon into Egg Groups based more primarily on type rather than the Pokemon's actual characteristics. Fairy and Psychic Pokemon are often sorted into Amorphous.
All in all, while one can make a compelling argument for Ralts and its line to be solely in the Human-Like Egg Group, the decision is ultimately Game Freak's. Whether or not they have an actual rationale behind their decision to also put the line in Amorphous is unknown and, frankly, unimportant.
Egg Group -- Bulbapedia
Gardevoir -- Bulbapedia