Pro-rata means that early investor reserves its right to participate in future financing rounds, up to such amount as to maintain their ownership share. YC did

Pro-rata means that early investor reserves its right to participate in future financing rounds, up to such amount as to maintain their ownership share. YC did not do it before, but will start doing it now. They do not want to be leading investor, however, b/c if they do follow-up investments in one company but not the other, that would signal other investors their preferences, and will make financing prospects of companies they did not subsequently invest in, difficult.