DD is singing Deck the Halls, but all her 'fa la la's are the last line

[BlackAmericanoNoSugar]

It's bizarrely more annoying than I would have thought possible.

She's moved on to murdering Santa Clause is Coming to Town, and has just given herself a round of applause.

I'd join in, even louder and so out of tune x

I once travelled half an hour on a bus behind a child who sang The Wheels on the Bus but never got further than Round and Round. I was screaming silently All Day Long! all the way.

Sorry to sound like an idiot, but isn't the 'fa la la' bit the last line?

You sing 'fa la la' after every line, the first two are the same, but the penultimate and last of the verse has a different emphasis and melody both to each other and the first two. She was singing the verse end after every line.