Oddity with extract microseconds?

Does anyone else find this odd:

mysql=# select extract(microseconds from timestamp '2005-01-01
00:00:00.123');
date_part
-----------
123000
(1 row)

mysql=# select extract(microseconds from timestamp '2005-01-01
00:00:01.123');
date_part
-----------
1123000
(1 row)

No other extracts include other fields. eg, minutes:

mysql=# select extract(minutes from timestamp '2005-01-01 00:10:00');
date_part
-----------
10
(1 row)

mysql=# select extract(minutes from timestamp '2005-01-01 10:10:00');
date_part
-----------
10

So how come microseconds includes the microseconds from the 'seconds'
field and not just after the '.'? And if it's supposed to include
'seconds', then why doesn't it include minutes, hours, etc.?

Chris