Strange behavior with i18n variables

Hi,
I try to i18n strings with message files as explained in the manual, however if I use similar names with different postfixes, the framework seem to confuse the variables.
 
E.g. in the example: 
'msg_pro_ab_1' => 'Hello',
'msg_pro_ab_1a' => 'Bla bla',
'msg_pro_ab_1b' => 'bye bye',
etc.

it shows only 'hello' in the all cases.


Anyone has an idea what could I check? It seems to be too trivial to be a bug.

Best Regards,