In iOS 9 why is some text from speech to text underlined in blue? When I dictate text to my phone, sometimes to avoid having to type it, some of it is underlined with blue dots.
It looks real pretty, but the purpose of this is lost on me: if these words are treated differently from other words, or if the underlining means something, I have no idea what it could possibly be.

At first, I thought that it was because during voice dictation, the software had understood a couple of different things and that I could then chose between different options, but that's not what it seems to be.
If I click the words that are underlined, it doesn't give me anymore options than when I click non-underlined words.

Maybe it's app-specific? Although it looks like it would be system-wide. The app used in this screen cap is What's App.
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.