Good point. It could very well be that the same components would see more stress on an engine with fewer cylinders, and that could play into the difference in the maintenance recommendations, assuming you give Volvo and Audi the benefit of the doubt and allow that the difference may be related to a technical reason and not different sales philosophies.
Related to what you mentioned, I remember reading an engineering study once showing that engines with fewer than 6 cylinders operating at low engine speeds, when analyzed for "instantaneous" torque at different points during the crankshaft's rotation, are observed to have very large amplitude cyclic variations in that instantaneous torque, including significant negative torque at certain crankshaft angles. The graph of torque on the Y-axis with crank angle degrees on the X-axis looked roughly like a sine wave that moved significantly above and below the average. This is apparently even more pronounced in diesel engines due to their higher compression. Engines with more cylinders had much lower amplitude variations in torque, and much smaller maximum negative torque values. Above a certain number of cylinders (it was either 6 or 8) it showed that there is no negative torque at all anywhere during the crank's rotation.
To my recollection that particular study regarded the need for complex dual-mass flywheels to mitigate damaging torsional harmonics being transmitted through the gearbox and drivetrain from large displacement, high output 4-cylinder diesel engines. But obviously all the same cyclic torque variation would be equally present at the other end of the crankshaft, and would be fed into the timing system as well. Since the phenomenon occurs in inverse proportion to the number of cylinders, one could see how the same belt might live an easier life on a smoother running 6 cylinder engine than on a 5 cylinder engine, even though the belt has two extra cam lobes and one extra injection pump lobe to drive on the 6.
Of course it is also worth considering that all of these preventive maintenance recommendations are by necessity only crude generalizations, since there are many more factors that will affect the lifespan of these components than just odometer miles and years of time. For example if an engine is used for a long distance highway commute with a majority of the miles done in top gear, the timing belt will do far fewer revolutions in 60,000 odometer miles than it would if you did the same 60k miles primarily in low speed, city driving with time spent idling in traffic and at stop lights. Seems easy to imagine that the belt on the city driven engine could see double or triple the number of rotations in a given road distance... Number of thermal cycles probably matters too, as well as ambient conditions, etc. So as the saying goes, your mileage may vary ....
