The delivery rate is the share of sent messages confirmed delivered to handsets; it depends on the route, the list quality and the destination, and it is the least useful number in a report when read alone.
Delivered divided by sent looks like a quality score, but it mixes what the provider controls (the route) with what the sender controls (the list) and what nobody controls (phones that are off). A 92% rate on a fresh list to Germany and a 92% rate on an old list to Nigeria say very different things.
Read it per destination and next to the failure breakdown: a rising share of unknown numbers is a list problem, a sudden drop on one country is a route problem, and absent subscribers are just phones that are off.
At SMSMeteor: delivery rate per route is what decides which route a destination uses; your report shows it per country with the reasons behind every failure.