Why does top output for cpu utilization differ from mpstat on smp systems

This can me reproduced by running the following commands:

# top
# mpstat

Notice, after you run the top command, press "1" to get a list of all cores. This will show the actual cup utilization for all the cores.

Example:

Top command run without pressing "1"
Cpu(s): 9.5%us, 12.0%sy, 0.0%ni, 77.9%id, 0.4%wa, 0.0%hi, 0.0%si, 0.2%st

Top command run after pressing "1"
Cpu0 : 2.0%us, 8.0%sy, 0.0%ni, 77.1%id, 12.6%wa, 0.0%hi, 0.0%si, 0.3%st
Cpu1 : 3.3%us, 7.3%sy, 0.0%ni, 89.1%id, 0.0%wa, 0.0%hi, 0.0%si, 0.3%st
Cpu2 : 18.2%us, 13.9%sy, 0.0%ni, 66.9%id, 0.0%wa, 0.0%hi, 0.0%si, 1.0%st
Cpu3 : 7.3%us, 12.6%sy, 0.0%ni, 80.1%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st

Output of mpstat command

[root@xen ~]# mpstat -P ALL 1
Linux 2.6.18-128.1.10.el5xen (xen.pnq.redhat.com) 05/23/2009

04:56:19 AM CPU %user %nice %sys %iowait %irq %soft %steal %idle intr/s
04:56:20 AM all 9.09 0.00 10.32 0.74 0.00 0.00 0.49 79.36 4929.70
04:56:20 AM 0 10.89 0.00 8.91 0.00 0.00 0.00 0.99 79.21 1289.11
04:56:20 AM 1 2.97 0.00 11.88 2.97 0.00 0.00 0.99 81.19 2083.17
04:56:20 AM 2 13.86 0.00 10.89 0.00 0.00 0.00 0.99 74.26 817.82
04:56:20 AM 3 8.49 0.00 10.38 0.00 0.00 0.00 0.00 81.13 727.72