Why does the OID jump by 3 when creating tables?

Hi all,

as I could not find the reason in the source code, can someone tell me why the OID counter jumps by 3 between two
createtable statements? 

postgres=# create table t1 ( a int );
CREATE TABLE
postgres=# create table t2 ( a int );
CREATE TABLE
postgres=# select oid,relname from pg_class where relname in ('t1','t2');
  oid  | relname
-------+---------
 16453 | t1
 16456 | t2
(2 rows)

These seems not to happen with other objects, e.g. namespaces:

postgres=# create schema a;
CREATE SCHEMA
postgres=# create schema b;
CREATE SCHEMA
postgres=# select oid,nspname from pg_namespace where nspname in ('a','b');
  oid  | nspname
-------+---------
 16459 | a
 16460 | b
(2 rows)

... or indexes:

postgres=# select oid,relname from pg_class where relname in ('i1','i2');
  oid  | relname
-------+---------
 16461 | i1
 16462 | i2

Thanks in advance
Daniel