Because some of the trains are formed of three "single ended" units- for instance, the C-Stock is formed of three pairs of driving-motor and unpowered-trailer cars. Gives the ability to split and change formations, just need to make sure you end up with a cab at both ends. Some other stock types are formed of two double-ended units- though some of these might have had cabs effectively "deactivated" (these aren't painted red) such as many of the A-Stock units.
Even the S8 and 2009 stock is technically formed of two single ended 4-car units, with the formation being able to be easily split in half as required on depot.
With 73 stock on the Piccadilly they needed six car sets for the main service and 3 car sets for the Aldwych shuttle. So as not to have a big gap between doors in the middle of a six car train you would get an UNDM (uncoupling non-driving motor car) on a single ended set to couple to a 3 car set.