It's really about the meaning of 'decide';
people say they are deciding something but they're not really.. They are discussing it or thinking about it and TRYing to decide. 'decide' means make your final choice, so 'deciding' doesn't really make sense, strictly speaking. But people say it and know it means 'trying to decide'.
So, it is wrong to say 'deciding', but people do - they think it means 'considering', which it doesn't!
:)