The routing was changed. The aircraft that was now scheduled arrived too late to get him out on time. The cause of his delay was no timely availability of a functioning aircraft, but availability of an aircraft that was functioning that arrived late to use for his.

Someone who wasn't on his inbound asks the agent why they are leaving late. They would say "We don't have an aircraft for your flight until later." An accurate and appropriate answer. Hence the delay code. Again, airlines reroute aircraft all the time. One cannot predict with much accuracy the routings of aircraft. In this case, the aircraft that was scheduled that day was non-functioning. If you ever looked at the scheduling of aircraft, in the 2-3 day window before a flight, aircraft routings change very frequently. So frequently that I would bet most aircraft routings are changed on average a few times in the last 3 days before a flight. Should they have said the plane was delayed for a mechanical/ Was the plane (the actual plane used) on the ORD-MCO having mechanical difficulties? No, it wasn't. How would that have been any more accurate?

Internally, we can (if skilled at "old school" techniques that are not taught anymore) tell it was a plane change, but again, since airlines don't publish their fleets routings, how would that info be needed by anyone more than the fact that the aircraft used arrived too late to get an ontime dptr for the ORD-MCO flight?

No smugness here, just a real world knowledge of the systems and methods used. Armchair quarterbacking, second guessing without knowledge or experience, or being a self assumed expert is no substitute for first hand knowledge of the system and methods.

I've had a career as a controller for UA (the person who codes the delays,) I know the reasons for the codes, I know the internal codes (a large book of different ones) that code the public explanations (only a handful of one are displayed, and they are "dummied down". What is displayed to the public isn't the reason for the internal code, those are used for many more reasons. The public info was correct, the internal info was correct. Trying to understand the system without knowing the internal processes will only lead to you being frustrated. Accepting the fact that his plane was delayed due to a late arrival, for whatever reason, is sufficient. What difference does it make to the customer in the long run, if the inbound was late becuase it took a wrong turn, got held for wx, for crew, for passengers, or whatever at an upline station, other than curiosity? If UA had to pay for hotels, they would have, as the internal system is what triggers that, not "late arrival of aircraft" (which covers a myriad of internal reasons.) What matters is there was no useable plane for him, and due to that his flight was delayed, and hence the code of "late arrival of aircraft" (or as we call it, a "late turn".)