Good question. I have thought of those numbers as the right ones for a long enough time now that I cannot remember exactly what source I would need to cite, but it is based on a published figure that I could probably find again. There are various different specifications that are published so you end up having to decide for yourself which one to trust. Sounds like you saw 80k or 8 years for your later model car. Volvo's original spec that they published for the early style diesels sold here in the US was 75k miles IIRC. There are probably others.
As best I can remember, where I saw the 60k/7 year spec quoted was for the old style 2.0L Audi 5 cylinder diesel engines. Those engines use exactly the same timing belt system as the D24 and D24T (and same water pump, idler roller, etc, the whole front end of the engine is perfectly identical in every way). I figured if Audi only trusted these exact same parts for 60k miles, I probably would only trust them that far also, even if Volvo believed an owner who liked to gamble might get a few more miles out of that belt.
Deeper dive into my reasoning on this: when Volvo first introduced these diesel engines, one of the selling points they advertised for them was the idea that they could require much less routine maintenance than the gas engine alternatives. That included very infrequent oil changes according to their original maintenance schedule (which was later updated and made less extreme). The original long oil change interval is known to have caused rapid wear and premature failures in many of these engines before the spec was updated, so they obviously pushed the service schedule beyond what the actual machinery could support in that area. I figured they might have done the same with the TB interval, wanting to publish as optimistic a figure as they could, especially knowing that the TB change would be far more labor and cost intensive for the diesels than for the gas Volvo engine.
Long story short, I suspect that the longer recommended interval for the Volvo application vs the Audi application with the same parts was not an engineering decision but rather a marketing decision. Since the consequences are so severe for a timing belt failure on this type of engine I feel that erring on the side of caution and going with the most conservative recommendations available makes sense.