It is odd, isn't it? axa, bab, cdc, ede, ded - instead of the usual rhyme scheme of aba, bcb, cdc, ded. efe, [... &c]. I've read that English writers of the terza rima do often vary the pattern on account of English's being a 'rhyme-poor' language (though word-rich), in comparison with Italian's being rhyme-rich (though word-poor). So maybe Bowra was following this tradition. It's possible too that B. in not folllowing the more rigid rhyme pattern, was trying to convey something of the fragmentary nature of the poem, or perhaps the metrical complexity of the original?