First of all...get it right Sony said "PlayStation Network has amassed‭ ‬17‭ ‬million registered ACCOUNTS,‭" not "USERS". There is a very distinct difference. One user can have many accounts. Please understand the English language be fore you make a comment. Second 21 million is "actual units sold to consumers" not "shipped to retail" like Microsoft reports. So in fact Microsoft has always inflated their numbers. There is almost 7 Billion people in the world which make 21 million an insignificant number.
So 21 Million is very easy to believe. Also in this economy it would be stupid to lie about sales the repercussions would be to great. So keep your conspiracy theories to minimum.