A real world example of a read-only variable which can't be recalculated on demand is your car's odometer.

Ignoring that in the real world, the DVLA might argue with that assessment. Clocking is still big business, though it is getting harder to do. :)

That is an example of both "Except for the rarely implemented concept of externally read-only attributes which are internally read-write." and an exception to "most times".

But even that belies the reality of (at least modern) real world odometers which can display in either miles or kilometres, but internally probably count output shaft revolutions. They might therefore be best modelled as an internal revolutions counter attribute with a pair of methods that apply an appropriate scaling factor to that attribute to produce the miles or kilometres figure.

Why not just count in miles and convert to kilometres on demand you might say. But back in the real world the same electronics in the dash are used in vehicles with multiple different drive trains. Eg. The BMW 3 series comes with 1.6, 1.8, 2.0, 2.5, 3.0 & 3.5 litres engines. You also have low-reving diesel and high-reving petrol engine options. Whilst the gear ratios will usually account for much of the difference in engine speed to road speed, they do not fit a rear axle capable of withstanding the rigours of 300bhp to the 120bhp 1.6 models, and the final drive ratios will be different. Adding to that, different models have different size rims and tyre profiles so the odometer has also to account for different rolling radii.

The upshot is, that they do not make a different odometers for each of these combinations of model to allow them to count directly in miles or kilometres, but instead count in some arbitrary revolutions of the input and then write different constants ("appropriate scaling factors") to ROM which are used to convert the counter to real-world measurements.

In other words, there are no getters or setters for the actual attribute; just methods to obtain calculated derivatives of that private attribute.