In comScore’s case, their MobiLens data comes from “an intelligent online survey of a nationally representative sample of mobile subscribers age 13 and older”. They don’t disclose the number of people surveyed, but you can bet it’s not a massive number. In NPD’s case, they survey 12,811 people.
Which numbers do you trust? Millions upon millions of actual sales reported in a legal manner by public companies or surveys of thousands of people?