>>33770339First and foremost, some wankery is required with this topic.
So, Pokemon are partially digital. The Pokeball condenses them in such a way that they can be transferred through a computer and more.
Therefore, it is logical to assume that when traded, pokemon are also broken down, and thus are prone to errors.
Pokemon that evolve when traded with an item do so by scrambling the difference between the pokemon and the item, and just fusing them. Except Porygon, who likely enters a deliberate "update receptive" state under similar circumstances.
Machamp and Golem are similar to Escavalier and Accelgor. Their features get scrambled, and you end up with Machamp walking off with an extra set of arms, while Golem leaves with a new head.
Haunter is a difficult to define pokemon, being composed of spiritual energy and gas, and what not, so when it is broken down, it is reassembled into a much more definable form, since that is the best the operation can do.
Kadabra is intelligent enough to recognize what is happening to it, and thus takes it upon itself to a bit of maintenance on its brain makeup.
Many of the later item evolutions obey similar merging principles, though the King's Rock is an odd duck.
Some of the later straight trades did not have these features for a reason I have no idea how to address.