Why does UPDATE STATISTICS WITH SAMPLE take longer then FULLSCAN?

We've been running UPDATE STATISTICS with FULLSCAN daily against databases that total 280GB.  The small databases are no problem, but the largest database takes over 3 hours.  Overall the total time for all databases ranges from 4 hours to 4.5 hours.  We decided to implement UPDATE STATISTICS WITH SAMPLE = 40 thinking this would cut down the Update Stats time dramatically.  That wasn't the case.  So far tests on two different servers showed using SAMPLE=40 taking longer then FULLSCAN.  Why would this run as long or longer then FULLSCAN?  FULLSCAN is supposed to be the same as SAMPLE=100.  

Thanks,    Dave