A few reasons...
1.It’s really a trick they changed the method used is the easiest way to explain it.
2. As you pointed out Seasonal adjustments.
3. For months now they put out numbers based on prediction and then a few weeks later or so they are adjusted downward to the actual numbers but unless you look for it people are led to believe the original numbers were fact.
4. They stopped counting those who fall off the unemployment roles.
I think these might help explain it further :)
http://www.techcrunch.com/2009/12/04/real-unemployment-17-2-percent-mint/
http://briansullivan.blogs.foxbusiness.com/2009/11/06/is-the-real-unemployment-rate-17-5/

