Repairing Siege & boat units

Repairing siege & boat units...

My normal thinking was that if a siege or boat unit uses armor plating then it is a non-human type unit so you need villagers or engineers for repairs.

Similarly if a siege unit uses medium armor as per some Norse & Celt units: log / stone throwers / rams etc
it is a human type unit so you need priest units for repair.

Now we have the Roman Scorpio.
This unit uses medium armor yet requires villagers or engineers for repairs ???

My expectation based on logic and playing other civ's was that Scorpio's would be repaired with healing priests i.e. the Clinicus.

Now logic goes out the window, here is the exception to the rule
This is intentional I presume?