Time difference between admission offers and rejections in US PhD Admissions?

Being that I am currently awaiting offers of admission/rejections for some of my PhD applications in the US, i regularly frequent the website http://thegradcafe.com, which, if you've never heard of it, has a forum where just about any subject relevant to graduate students is being discussed, and, most importantly, maintains a database of admission/rejection dates for various programs at various schools (see http://ift.tt/GRjgCG).

While discussing with other members of the forum and browsing through the database myself, I have noticed the following pattern: For many PhD programs, year after year, all the offers of admission (the ones added to the database, anyway) will be made on a single day, and then, some time later (often 2,3 weeks or even almost a month later), all the rejection notifications are made.

Of course, it should be said that this "data" is fairly unreliable, since it is all added anonymously and most likely very fragmentary, but assuming this is actually a trend, here is my question:

What is the explanation for such a long delay between offers of admission and notifications of rejection in (some) US PhD admissions?

The possible explanations I have in mind are the following:

1. The candidates that are assuredly admitted are sent offers as soon as possible, and then the admission committee takes a lot more time to choose a few more candidates within the (probably very large) sample of not-so-surely admitted students. (This could possibly not be reflected in the Grad Cafe's database, as very few candidates could be admitted this way)


2. Universities wait to see if some of the strong applicants will answer positively or negatively to their offers before deciding to reject other applicants (but then, why not waitlist them in the mean time).

However, I would be very interested in hearing about anyone who has been in (or knows about) an admission committee that operates like this, and the reason behind this.