Curious about dead rows.

I am doing lots of INSERTs on a table that starts out empty (I did a
TRUNCATE on it). I am not, AFAIK, doing DELETEs or UPDATEs. Autovacuum is
on. I moved logging up to debug2 level to see what was going on, and I get
things like this:

 "vl_as": scanned 3000 of 5296 pages, containing 232944 live rows and 1033
dead rows; 3000 rows in sample, 411224 estimated total rows

A little later, it says:

"vl_as": scanned 3000 of 6916 pages, containing 233507 live rows and 493
dead rows; 3000 rows in sample, 538311 estimated total rows

(I suppose that means autovacuum is working.) Is this normal, or have I got
something wrong? Why so many dead rows when just doing inserts? It is not
that I think the number is too high, considering the number of rows in the
table at the point where I copied this line. It is just that I do not
understand why there are any.

I could easily understand it if I were doing UPDATEs.

postgresql-8.1.9-1.el5

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