Deadlock error!?

I got the following error while running some php code:

Warning: pg_exec() query failed: ERROR: deadlock detected in
/www/htdocs/jc/import/import_orders.php on line 189
CONNECTION: could not execute query (update invoices set
total_price=(select sum(price) from invoice_li where invoice_id=id))

This happened while the PHP code was running and and on the console I
did this query:

psql TMP -c "update invoices set payment_rcvd=true"

The PHP code was executing SQL inside a transaction and the total
running time is about 1 minute. Why a deadlock?

Whatever is happening inside a transaction is given a snapshot of the DB
and should be oblivious to other queries no?

Jc