My guess is she liked his singing and decided to give him the anecdote. Probably there was some admiration of their fight, too.

I was confused about that tho. Since Lancel confronted Cercei in the first episode(?) of the season at Tywin's funeral. Littlefinger was still in the Veil. So was he talking about Lancel & taking more credit than is his, or does he have another ace up his sleeve? Especially since if it's revealed that Cercei's…