<p>There are people out there who accepted or already bound to some of these schools.</p>
<ol>
<li> Sports recruits - They are eliminated from consideration by other Ivies when they are committed and admitted to the school of their choice.</li>
<li> Legacies usually have a high percentage of yield.</li>
<li> Asians have a high percentage of yield - in 2011, Harvard’s admits vs final pool had a difference of 2% (went from 18 to 20% with a yield of 90+%).</li>
</ol>
<p>So harvard early admits are probably at 50-60% committed already even if some have not actually accepted. Harvard knows precisely what to expect even if they don’t tell the general public.</p>