I always think of Kate Middleton when I see this phrase.
If you look at photos of her at public events these days, she is always smiling and looking so happy to be there.
Her face is never 'at rest'.
She's clearly been coached to never let her face fall into repose, precisely because her natural resting face falls into a slight frown, making her look unhappy, disapproving, bored, whatever.
I mean this is normal - it's how most of is look when we're going about our business, not really concentrating.
I've really started to notice it.
She always looks smiley, camera-ready, happy to be there.
It must take SUCH an effort!