Very surprised to see the Netherlands at 90%, New Zealand at 86%, Sweden at 88%, Japan at 84%, and Australia at 79%. Seems incredibly low for countries with reasonably high quality telecom infrastructures.

New Zealand, Australia, and to a lesser extent Sweden all have vast areas with very low, near zero population densities. Could that be impacting the deliverability?

Absolutely. Outside of the metro areas you start to lose service. Smaller towns sometimes lack service altogether on some providers. Anywhere bush/mountain you'd be very very lucky to get service (of which there is a lot).

Near 50%? It's 38% in Vietnam.