Coupling trailer at correct height
The trailer contacts the tractor's fifth wheel plate on the back half, and the tractor firmly slides under the trailer.
Coupling trailer that is too high
The trailer doesn't contact the tractor's fifth wheel plate until the front/second half, and the plate is simply "slapped" straight as the tractor moves under the trailer. In this case, coupling was still successful, however if the trailer were to be much higher the tractor's fifth wheel may not catch the trailer's kingpin, at which point the driver stands a chance of backing the tractor's cab into the front of the trailer, or at least having the kingpin "caught" in front of the fifth wheel.
To prevent this from happening, the trailer's landing gear should be cranked to lower the trailer before the tractor is backed under the trailer. This would allow proper coupling as shown in the first video.