Why Postgres default FILLFACTOR for table is 100 and for Index is 90.
Although Oracle is having completely different MVCC architecture, it uses default 90 for table and 100 for Index (exact reverse of Postgres)
Postgres blocks needed more spaces for row update compares to Oracle (because Oracle keeps buffer space only for row expansion, whereas Postgres need to create new versioned row). As I see Postgres is more suitable for OLTP workload, keeping TABLE FILLFACTOR value to 90 is more suitable rather than stressing to save storage space. Less FILLFACTOR value will be useful to make UPDATEs as HOT applicable as well and that is going to benefit new Postgres adopting users who are initially not aware of such setting and only realize this later when VACUUM are really running long and Indexes gets bloated. .
Other side Index FILLFACTOR makes sense only for existing populated tables and for any row (new INSERTs or INSERT coming through UPDATEs), it can fill the block above FILLFACTOR value. I think 100 default make more sense here.