EXPLAIN BUFFERS: dirtied

Hi,

In EXPLAIN (ANALYZE, BUFFERS) for a SELECT query, I see the following
statistics under an Index Scan node:

Buffers: shared hit=8357288 read=6165444 dirtied=44820 written=5590

As far as I understand, that's the statistics for accesses to shared
buffers during the query:
- hit = required page was already in shared buffers
- read = required page was not in shared buffers, and was loaded from
disk (from filesystem cache)
- written = while loading the required page, there was no free space for
it in shared buffers, so some other dirty page was evicted from shared
buffers and was written to disk (to filesystem cache), to free some
space and to load the required page

But what is "dirtied" statistics? When a SELECT query could make pages
dirty?

Regards,
Vitaliy