Why is the Earth moving away from the sun?

Short answer: tidal friction, the same reason the moon is receding from us.

But there’s a “what the?” in the story.

[T]he sun-Earth distance has been pegged with remarkable accuracy. The current value stands at 149,597,870.696 kilometres.

Having such a precise yardstick allowed Russian dynamicists Gregoriy A. Krasinsky and Victor A. Brumberg to calculate, in 2004, that the sun and Earth are gradually moving apart. It’s not much – just 15 cm per year – but since that’s 100 times greater than the measurement error, something must really be pushing Earth outward.

No error is reported, but presumably the error is in the last digit of the value, 0.001 km. One one-thousandth of a kilometer is a meter. 15 cm is smaller than that. If the error is as was reported, it should be a few millimeters. If that’s the error, why isn’t the distance known and reported to that level?