You are viewing the aircraft planning at a certain point in time, which means that the entire image is not displayed.
Most likely the plane that was going to fly UA263 was delayed or could not make it to the ORD in time for another reason.
To do this, the airline had to find another plane, and the first one they had was the UA1140. This clearly resulted in a delay for UA263, but this delay would have been shorter than the delay if they had been waiting for the originally planned aircraft instead.
The "inbound" flight is only displayed on the United website at the desired time. At this point they have already made the change, so you see an "impossible" schedule. For example, if you checked yesterday, you would probably have seen another plane flying that flight – with an appropriate buffer between flights.
In practice, these "swaps" are generally far more complex than described above and can result in multiple aircraft being moved between flights to minimize delays across all flights. This is especially true if bad weather or another problem causes several flights to be delayed.
To update: I dug a little, and this is indeed an exchange of several planes. The original release was UA795 from Windsor Locks, CT, which was delayed by more than 9 hours. To overcome this delay and minimize the impact on future flights, United exchanged multiple aircraft over multiple flights. Your flight was originally scheduled to use the aircraft arriving as UA1573, but this flight was reassigned to another flight, the aircraft of which was again assigned to another flight to cover the delay on UA795.
The end result of this is that some flights are a little delayed (e.g. if you arrive 25 minutes late). However, this is better than some flights with a delay of> 8 hours each, as would otherwise be the case.