increase of xact_commit vs txid_current

[reg_pg_stefanz]

Hi,

I am confused, the documentation says for pg_stat_database
   xact_commit      Number of transactions in this database that have 
been committed
and somewhere else
    txid_current()    get current transaction ID, assigning a new one 
if the current transaction does not have one

I would have naively expected txid_current() be more or less in line 
with xact_commit,  or  increasing faster as txid_current() should be 
global but xact_commit is per database.

However xact_commit seems to increases faster than txid_current(),  what 
am I missing? Are there commits that do not increase the xid number?

Thanks
Stefan