I actually think their bot gets lost.
Cuz one site of mine bounced in and out several times. Now it is ranking #1 for the keyword I wanted and has stayed for awhile.. ( but the site could be gone tomorrow and come back next day ) I don’t get to excited about MSN, since I have seen this a lot with quite a few of my sites.
It is also possible your site is still in there just now under a totally different keyword that may not get the traffic you want.