surprised to find bloat in insert-only table

I recently set up partitioning on a table that sees heavy insert
traffic. There are never updates or deletes, we just drop the partitions
later.

It's my understanding that bloat can only appear through updates or
deletes, but these partitions are reported to have significant bloat in
them. Where else can this come from and how I can I reduce it?

I'm using a "bloat" view with 9.0.3 which might the same as this one:

https://wiki.postgresql.org/wiki/Show_database_bloat

This shows that each index on the table has over a gig of bloat:

select distinct(iname), wastedbytes, wastedsize from bloat where
wastedbytes > 0 order by wastedbytes DESC limit 20;

And this query also shows the same amount of bloat in the table itself:

select distinct(tablename), wastedbytes, wastedsize from bloat where
wastedbytes > 0 order by wastedbytes DESC limit 10;

Mark