The readers only come in on one day a week. They go through the scripts and take the ones they like away to read. The rest get returned without feedback. That is the sift. Every two weeks they come in to discuss the scripts they've read.
Basically, you CAN get your script back unread and un-critiqued even if it's f**king marvelous. The reader has to be interested in it enough to take it away. I had a script fail the sift which was also read by a producer who said it was much better than the stuff the writersroom got. They didn't seem to think so.
What is most annoying is that they claim their remit is to nurture talent - one of the reasons they will read stage plays. This is obviously not the case.