Text typing functionality seems to change by itself and at random - double tap letter/navigate to letter then lift. Why is it changing & how do you select the desired setting?

When using voiceover on an iPod Touch 5g (iOS 6.0.1) the text typing functionality seems to change by itself and at random. Sometimes you have to tap each letter twice to type that particular letter and other times it switches so that you just navigate to the desired letter and lift your finger off it to type that particular letter. Why is this happening and how can I pick the one I prefer? Is there a setting I'm not seeing somewhere within Accessibility? Might there be a gesture that's switching the setting by accident? This is to help my father who is blind and is becoming confused and concerned about the random switching of the typing behaviour. Many thanks.