O H/sec with PhpBB

Hi,

Considering thousand of PHPBB hashes.
I launched a dictionnary attack, I got ~400 H/sec.
GPU : 7970.

I then lauch a bruteforce attack on digits :


Session.Name...: oclHashcat
Status.........: Running
Input.Mode.....: Mask (?1) [1]
Hash.Target....: File (phpbb.txt)
Hash.Type......: phpass, MD5(Wordpress), MD5(phpBB3)
Time.Started...: Sun Jan 12 22:02:18 2014 (1 min, 1 sec)
Time.Estimated.: 0 secs
Speed.GPU.#1...: 0 H/s
Recovered......: 0/13681 (0.00%) Digests, 0/13681 (0.00%) Salts
Progress.......: 44650/136810 (32.64%)
Rejected.......: 0/44650 (0.00%)

1 digit : 0 H/sec. Why 0 H/sec ?

Few minutes after :

Session.Name...: oclHashcat
Status.........: Running
Input.Mode.....: Mask (?1?1) [2]
Hash.Target....: File (phpbb.txt)
Hash.Type......: phpass, MD5(Wordpress), MD5(phpBB3)
Time.Started...: Sun Jan 12 22:02:18 2014 (30 secs)
Time.Estimated.: Sun Jan 12 22:08:41 2014 (2 mins, 44 secs)
Speed.GPU.#1...: 1 H/s
Recovered......: 0/13681 (0.00%) Digests, 0/13681 (0.00%) Salts
Progress.......: 219300/1368100 (16.03%)
Rejected.......: 0/219300 (0.00%)

2 digits : 1 H/sec.


Etc... 3 digits : 10 H/sec ; 4 digits 21 H/sec..

Am I doing something wrong with the command line ?
Why dictionnary attack gives 400 H/sec but simple digit bruteforce is slower ?
I tried changing -n and -u but this is the same.

Thanks.