I'm not sure that that's the reason, actually. The reason is that not everyone who suffers from disease X necessarily goes to get treatment for disease X which would be the only way in which that case of disease X could be reported.

So, for example, if it's the case that only 1/100 people who get salmonella have symptoms severe enough to cause them to go for treatment, then the _estimates_ of salmonella will be 2 orders of magnitude _higher_ than the _reported_ cases.