The feminization process is not perfect and neither are seeds.
Seeds being living organic being are subject to slight variations in genetics, leading to the odd plant showing some signs of imperfect plants such as hermies and other mutations.
It will not be caused by temps, I have grown plants outdoors for years that get subjected to temperature fluctuations from 10 degrees Celsius up to 48 degrees Celsius and never had any problems with hermies.
Hermaphroditism is primarily a genetic condition, but can be induced by either chronic stress and/or sustained stress. A few male bananas during late flowering can occur without any stress factors and is a response to being without seed, and is a survival "instinct".
Hope this helps,.............
Organoman.