Tom, 1. sometimes alternative form of the same case eding are used. Namely feminine istrumental sg. -ою. (-ой in speech). The reason is mostly just that the poet needs an additinoal syllable here:) We may use this '-ою' in speech to imitate poetic style) 2. there ARE some alternative non-standard case forms in Russian proper. cf. из лЕса vs. Из лесу. (capital for stress). Poets are more likely to use non-standard ones. The sound more 'true', more 'Russian', more 'folkish' etc. They are older. Wherever you have a choise, 'standard' one will be more 'boring'.