Inconsistent "position" numbering for keys in "system_schema.columns"

cqlsh> CREATE TABLE test.table1 (key1 text, value1 text, value2 text, PRIMARY KEY(key1));
cqlsh> SELECT keyspace_name, table_name, column_name, kind, position FROM system_schema.columns WHERE keyspace_name = 'test' and  table_name = 'table1' ;

 keyspace_name | table_name | column_name | kind          | position
---------------+------------+-------------+---------------+----------
          test |     table1 |        key1 | partition_key |       -1
          test |     table1 |      value1 |       regular |       -1
          test |     table1 |      value2 |       regular |       -1

cqlsh> CREATE TABLE test.table2 (key1 text, value1 text, value2 text, PRIMARY KEY(key1, value1));
cqlsh> SELECT keyspace_name, table_name, column_name, kind, position FROM system_schema.columns WHERE keyspace_name = 'test' and  table_name = 'table2' ;

 keyspace_name | table_name | column_name | kind          | position
---------------+------------+-------------+---------------+----------
          test |     table2 |        key1 | partition_key |       -1
          test |     table2 |      value1 |    clustering |        0
          test |     table2 |      value2 |       regular |       -1

cqlsh> CREATE TABLE test.table3 (key1 text, value1 text, value2 text, PRIMARY KEY((key1, value1)));
cqlsh> SELECT keyspace_name, table_name, column_name, kind, position FROM system_schema.columns WHERE keyspace_name = 'test' and  table_name = 'table3' ;

 keyspace_name | table_name | column_name | kind          | position
---------------+------------+-------------+---------------+----------
          test |     table3 |        key1 | partition_key |        0
          test |     table3 |      value1 | partition_key |        1
          test |     table3 |      value2 |       regular |       -1

cqlsh> CREATE TABLE test.table4 (key1 text, value1 text, value2 text, PRIMARY KEY(key1, value1, value2));
cqlsh> SELECT keyspace_name, table_name, column_name, kind, position FROM system_schema.columns WHERE keyspace_name = 'test' and  table_name = 'table4' ;

 keyspace_name | table_name | column_name | kind          | position
---------------+------------+-------------+---------------+----------
          test |     table4 |        key1 | partition_key |       -1
          test |     table4 |      value1 |    clustering |        0
          test |     table4 |      value2 |    clustering |        1