problem in sql - sum()

Hi. I have a problem identified.
In select :

select data_carteira, sum(valor_carteira) from carteira where
data_carteira between '2016-01-01' and '2016-02-18'
GROUP BY data_carteira order by data_carteira

return a values from month. In the day 18, return the value 82.915.213,14.

Now, if I use the select :
select data_carteira, sum(valor_carteira) from carteira where
data_carteira='2016-01-18'
GROUP BY data_carteira order by data_carteira

return 103.629.305,96.

It's very diferenty.
Only change is date.

The version in use is :
PostgreSQL 9.0.8 on x86_64-redhat-linux-gnu, compiled by GCC gcc (GCC) 4.6.3 20120306 (Red Hat 4.6.3-2), 64-bit

Please, helpe me.

Thanks,

Ivan Rodrigues
Brazil - São Paulo