cmin increments by 2 except in 7.4?

I've noticed that in PostgreSQL 7.4, successive commands in a
transaction get cmin values that increment by 1, but in other
versions cmin increments by 2.  Example:

CREATE TABLE foo (x integer);
BEGIN;
INSERT INTO foo VALUES (1);
INSERT INTO foo VALUES (2);
INSERT INTO foo VALUES (3);
COMMIT;

Results in 8.0.1 (same in 8.1devel, 7.3.9, 7.2.7):

SELECT xmin, cmin, x FROM foo;
 xmin | cmin | x
------+------+---
 7112 |    1 | 1
 7112 |    3 | 2
 7112 |    5 | 3
(3 rows)

Results in 7.4.7:

SELECT xmin, cmin, x FROM foo;
 xmin | cmin | x
------+------+---
  856 |    1 | 1
  856 |    2 | 2
  856 |    3 | 3
(3 rows)

What is 7.4 doing differently than the other versions?

--
Michael Fuhr
http://www.fuhr.org/~mfuhr/