.....I suppose it made sense once, and no-one has updated the datasheet since then...

It makes sense today. And there is no reason to hit the exact number.

Two constraints:

The /ratio/ of the two resistors gives the desired output.

The chip needs enough bleeder current to swamp the internal current flowing at the ADJ pin. I(adj) may be as much as 0.1mA. The sheet suggests you swamp this 52 times with 5.2mA in the divider. So the maximum "good" value of the upper resistor is 240 Ohms. A perfectly standard value, especially when these chips cost $3 each.

No 240 in your drawers? Use 220 and fudge the bottom resistor by 22/24. Or use 100 Ohms and 0.4166 the bottom resistor, if you can afford the 13mA idle current.