Complex sentence: Note that the date is that of manufacture, not the date the vehicle was registered in Portugal-a significant number of plates have a year and month seemingly out of tune with the alphanumeric sequence, but these are the examples of imported used vehicles.
Split into simpler sentences: Note that the date is that of manufacture, not the date the vehicle was registered in Portugal. Some vehicles carry plates that have a year and month seemingly out of tune with the alphanumeric sequence, and the reason for this is that these are imported used vehicles.