Why are there sequence gaps in auto generated project numbers?

A question came up yesterday as to why there are gas in the sequence of project numbers that are automatically generated.  At first I thought, oh, Oracle must be using a sequence for generating the number and it must be aging out.   But after doing some digging it looks like the number is set in table pa_unique_indentifier_control and incremented by 1 when a new project is created.   Found this code below:

  SELECT NEXT_UNIQUE_IDENTIFIER

  INTO uniqueid

  FROM PA_UNIQUE_IDENTIFIER_CONTROL

  WHERE TABLE_NAME = x_table_name

  AND NEXT_UNIQUE_IDENTIFIER IS NOT NULL

  FOR UPDATE OF NEXT_UNIQUE_IDENTIFIER;