.. but then why when I review my score, does it display
Because that wasn't Question 15! I don't want to share question/answer specifically as it spoils it for others. It was a multi-select answer, and you only selected one option.
I can see that causing some fun...
I think there are crossed wires Paul.
In you pic, you are at the top (showing Q1) and it shows a green tick to indicate you answered it correctly.
If you scroll down to Q15, you should see a red x
against your selection.
So when Joe said "Because that wasn't Question 15!" he means your screenshot was of Q1.
EDIT
Mind, the "Score 1/1" is confusing me. Might be a bug with the LearnWords application. Or perhaps browser issue? (Safari? Chrome? FF?)