Oh wow, how did you even notice that!? That has to be it.A long-shot, but...
One of the types of each Pokemon in this set are weak to one of the types of the next Pokemon not related to its evolutionary line.
Xatu --> Mareep (Flying is weak to Electric)
Flygon --> Cacnea (Ground is weak to Grass)
Weepinbell --> Tentacool (Grass is weak to Poison)
Deino --> Larvesta (Dark is weak to Bug)