The sentence I'm reading goes as follows:

"за что их называли 'женский батальон' а сердюкова 'командиром женского батальона'"

If both "женский батальон" and "командиром женского батальона" are being acted upon by the verb "называли" then why is one nominative and one instrumental? Shouldn't they be the same? It is like saying "Маша читал книгу, a Федор статьёй" I don't get it. Someone please help.