Average time spent on site

Average time spent on site in Omniture site catalyst and Average time spent on site in Google Analytics are showing extremely different numbers. Please give us a feedback of why they are so much different. 

Omniture Sitecatalyst : Average Time spent on Site (07/24/2011) : 15.33

Google Analytics : Average time spent on site (07/24/2011) : 4.37

Thanks, 

Gan