How to do a real-brute-force ?

[epixoip]

No. You're not specifying a mask, so it's using the builtin mask as documented on the wiki:

http://hashcat.net/wiki/doku.php?id=oclh...ult_values

To do a "full lower-case alphabet brute-force from 3 to 12 character" you'd add the mask "?l?l?l?l?l?l?l?l?l?l?l?l" to your current command. To do a "full brute-force operation" with all possible characters you'd use the "?a" builtin character class instead of "?l".