The short answer is that it’s not perfect. I think another explanation is that this fare does exist, but the airlines have allotted zero seats to it. Be sure to try a bunch of sites, just in case it slips through on one of them. You can also cross-check with ITA.

Another short answer is that FareCompare results are a bit like new car sales ads.

Airlines occasionally file (that is essentially advertise through the booking engines) fares that they have no intention to sell. For some reason it happens much more often on international routes then on domestic ones. It is not uncommon to see an airline advertising fares on a route it does not even fly.

FareCompare constructs Getaway maps based on whatever fares airlines advertise. It simply unfeasible to check seat availability, say from Denver, to every city in Europe on every day during entire month. Travelocity Dream Maps, by the way, are constructed in exactly same way.

I would not count on a seat to “slip” through when it never existed. Instead try ITA month-long search with different destination airports. This takes more time but ITA checks seat availability for every routing it displays.

The seats that never existed don’t slip through, but sometimes some combination, whatever it is, does slip through and is a low-priced anomaly not found on any other site.
 
In any event, it looks like you have MSP-SNN for $846 total, on Delta, for real, for June 13-28, if you want it. With a little experimentation, you would probably get a higher airfare, but you could open-jaw it with another Delta destination.
 
Another place to check that sometimes has good deals is 1800flyeurope.com .

I dunno what’s up with it, but I’ve noticed the same thing before. I am wondering if they now list consolidator fares from sites they partner with to show the lower realm of possible fares during a season, but like all inventory, is subject to sell out. Or it could just be a mistake, but I don’t think Farecompare or any other source like ITA et. al. are being deceptive—they rely on data provided and updated daily by the airlines they list.

Since you’d probably shop around anyway… I do see $859 to Frankfurt your dates, found via Bookingbuddy to Smartfares.

Also, since Icelandair serve MSP, and since they sometimes have pretty decent fares for departures before mid-June, you might sign up for their sale alerts, and watch their site during December. They have in past years offered good value “gift certificates” which were pre-paid fares for upcoming travel seasons at very good prices. If I recall… MSP-CPH around $555+tax (round trip) last year. Use our search feature top of this forum to search Icelandair MSP or similar for posts from early 2006—I remember posting about them. Will give you an idea of possible fare range if Icelandair decide to repeat a similar offer this year.

Don-
I just checked it out and that Smartfares ticket to Frankfurt for $859 appears legit and is by far the best I’ve seen.  I’m tempted just to book it, but I think I’ll wait about another month to see if Icelandair has a sale.  I did the search and saw the actual fare was $510+tx into Copenhagen.  In the end, that ends up being pretty close to the Frankfurt ticket.  Thanks again!