It's definitely possible that there are more infected then reported. The issue is Italy is testing only people who are suspected to be sick in the first place.

Most of the people who got covid, are asymptomatic thus they probably won't be tested and added to the count.

This means that 10% is taken from a population of people who are both sick and symptomatic.

Since China is testing asymptomatic people as well, they should have a more precise ratio.