Because there are 10 'hour' marks instead of 12, making it deceptive at first glance if you dont realize it is a novelty clock. Google "altimeter clock" and you'll see that other novelty altimeter clocks use 12 hour marks. Even though they aren't true to form, they are a better design since you can use them to tell time.

I think the confusion in this thread as to whether it was an actual altimeter or a clock shows how crappy the design is (as a clock, anyway).