9.0 EXPLAIN Buffers: written=nnnn

Do EXPLAIN ANALYZE:

. . . only showing the bottom node . . .
->  Seq Scan on YYYYY
(cost=0.00..37962.29 rows=876029 width=40)
(actual time=16.728..92555.945 rows=876002 loops=1)
Output: foo, bar
Buffers:
  shared hit=146
  read=29056
  written=2325   !!!!!

"Total runtime: 375542.347 ms"


Then Do EXPLAIN ANALYZE again:

. . . only showing the bottom node . . .
->  Seq Scan on YYYYY
(cost=0.00..37962.29 rows=876029 width=40)
(actual time=0.192..2972.127 rows=876002 loops=1)
Output: foo, bar
Buffers:
  shared hit=210
  read=28992

"Total runtime:  32913.884 ms"


In general, can you tell me why [written=2325] is displayed by the first EXPLAIN, but not the second EXPLAIN ?
The query is a SELECT, so what is getting "written" ?

Note that both EXPLAINs have the exact same plan. Bottom up is: Seq Scan on YYYY, Hash, Seq Scan on XXXX, Hash Join,
HashAggregate

I am wondering if it is a clue as to why the "actual time" is so different from one run to the next.

The query is 2 tables joined, with a group by:
select
  sum (case when X.hid is null and not Y.cntr = '0' then 1 else 0 end) as colp
from
  xxxxx             X
  inner join yyyyy  Y on Y.coln = X.colm
where
  X.some_id = 'aeiou'
group by
  X.some_type

Thanks,
-dvs-