In iOS 9 why is some text from speech to text underlined in blue?
You are actually correct as the highlighting appears for words that the system doesn't think that it fully recognized. If it can determine similar words to suggest than when you tap the segment of text, it will provide those alternates - to a remarkable degree of accuracy at times. However, if it cannot determine other possibilities, the text simply becomes highlighted when you tap it so that replacement of the erroneous word becomes very easy: just tap the word and type the correct one.