By limiting the number of invitations that they issue in each draw, they both make their system more selective by only inviting the most qualified candidates, as well as prevent themselves from becoming overburdened with applications during a given period of time. That way they can ensure that they can process the applications that they receive in a timely manner. They also only want to invite a certain number of people per year, so it is most fair if they spread their desired number of invitations out throughout the year so that they offer highly qualified candidates opportunities regardless of when they apply. Any selective system is designed so that they receive more applicants than they will approve. It is very logical.