Half of Reddit posts are ignored the fist time they're posted

From the report "In our dataset, 9,370 of the 172,030 links submitted to Reddit became popular (5.4%)." where popular means it was on the most popular page at some point.

They crawled every 10 seconds for 17 days, so found about 10.000 submissions/day. 10.000 a day! No wonder some post are ignored, that sounds unmanageable.