welshwheels, it's quite common, it's refered to as a "short" 12hr, the same format is also used at TwentyFour12, BrightonBigDog and 24hrs of Exposure to name a few.
You still get all the drama of trying to squeeze an extra lap in, and tactics can become even more exciting for those who are running close; imagine finishing at 6:01 and it not counting!
The idea is to give the event a "clean" ending, which is far, far better as it stops the slowest rider sprinting to get in then going out again with 1min to go and doing a 2hr lap, which would mean a MASSIVE wait for everyone until results could be finalised and podiums happen etc. This means that this format is getting quite popular. Good PA/timing systems also keep the crowd informed who is running it tight so makes for a very, very exciting finish for the last few minutes and there's oten a count down with 60seconds to go as you see some desparate rider appear on the horizon sprinting to get in, very dramatic! Mayhem, for example, is always a >25hr race with the hugest wait for the podium ceremony by which time lots of people have gone home, which is always a shame.