I think it might be the way teenagers talk, but not the way they actually think - at least not when they stop to think properly.
I have tried to come up with a justification for Nicola which goes something like this: Having only thought about Mrs Dodd's death from the p.o.v. of Rose and Chas, she has to do an instant re-think when her mother points out that Mrs Clavering is mourning two daughters. Out of embarrassed guilt that she hasn't even thought of that, Nicola says something silly without really thinking it through.