I've had personal experience with this. The first dojo I belonged to had kyu ranks from 7th through 1st, and I double promoted twice. The second dojo (lost all rank mind you and started afresh) I was triple promoted on a test, and just awarded a rank one day (came into the dojo and found people congratulating me on my promotion-odd that was). All were kyu grades; none were expected.
Don't think it is a big deal, or out of the ordinary. If you are training a good deal, your teacher sees you a lot and knows where you are. My teacher seemed to like the element of suprise, and I certainly had to go through the ringer of testing most times (and usually with a good deal of notice). Actually rather like the suprise promotions specially now that I am preparing for my (most likely) final test; and stressing inordinately about it!