Most critics view Owens' poem "Anthem for Doomed Youth" as a mixture of the English and Italian sonnet forms-a hybrid. The poem, as you correctly noted, begins as an English sonnet with an ABAB CDCD rhyme scheme. An Italian sonnet's rhyme scheme is ABBA ABBA. However, instead of continuing the English sonnet form of EFEF, Owens makes one subtle change: the sestet begins with EFFE, which as you see is more reminisicent of the Italian rhyme scheme in the octave. Owens completes his sonnet with a couplet that typifies once more the English sonnet.
In other words, the English sonnet form is ABAB CDCD EFEF GG.
The Italian sonnet form is ABBA ABBA CDE CDE.
Owens' form is ABAB CDCD EFFE GG.
Variations of this type are not unusual in sonnets. Even though they may vary in rhyme scheme, the constant elements are the 14-line length and the meter being iambic pentameter.