The number of infected is way off, which will throw everything else off. You can't figure out mortality rates, infection rates, etc if you don't have an accurate number to start with. Garbage in, garbage out.
The tests have a high false negative rate, which is one reason some countries are not testing everyone with mild symptoms. The test doesn't give you any useful information in that situation: if it comes back positive you need to self-isolate, but if it comes back negative and you have symptoms you still need to self isolate.