My Ubuntu 10.04 Lucid virtual machine is saying it's at 100% CPU usage... but all I'm running is Thunderbird. According to top, CPU usage should be ~25.9%... How do I interpret this conflicting output from top?

top - 13:55:26 up  3:35,  4 users,  load average: 3.03, 2.59, 2.48
Tasks: 178 total,   1 running, 177 sleeping,   0 stopped,   0 zombie
Cpu(s): 16.0%us, 79.7%sy,  0.0%ni,  0.0%id,  0.0%wa,  1.3%hi,  3.0%si,  0.0%st
Mem:    509364k total,   479108k used,    30256k free,     3092k buffers
Swap:  2096440k total,    58380k used,  2038060k free,   225116k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND                                                                                                                                    
 7708 jnet      20   0  480m 109m  17m S 18.4 22.1  21:59.14 thunderbird-bin                                                                                                                            
 4615 jnet      20   0  5488 1268 1040 S  2.3  0.2   5:00.03 nx-rootless-ses                                                                                                                            
 7124 jnet      20   0 56688  27m 4812 S  2.0  5.5   6:35.09 nxagent                                                                                                                                    
 6724 nx        20   0  9628 1400  636 S  1.6  0.3   3:26.59 sshd                                                                                                                                       
30106 root      20   0  2544 1236  908 R  0.7  0.2   0:00.33 top                                                                                                                                        
   19 root      20   0     0    0    0 S  0.3  0.0   0:22.45 ata/0                                                                                                                                      
   38 root      20   0     0    0    0 S  0.3  0.0   0:05.53 scsi_eh_1                                                                                                                                  
  345 root      20   0     0    0    0 S  0.3  0.0   0:04.72 kjournald                                                                                                                                  
 1719 root      20   0  3260 1192  944 S  0.3  0.2   0:17.36 vmware-guestd                                                                                                                              
    1 root      20   0  2804 1356  940 S  0.0  0.3   0:01.99 init                                                                                                                                       
    2 root      20   0     0    0    0 S  0.0  0.0   0:00.01 kthreadd                                                                                                                                   
    3 root      RT   0     0    0    0 S  0.0  0.0   0:00.00 migration/0                                                                                                                                
    4 root      20   0     0    0    0 S  0.0  0.0   0:00.15 ksoftirqd/0                                                                                                                                
    5 root      RT   0     0    0    0 S  0.0  0.0   0:00.00 watchdog/0
...

Specifically I'm referring to the fact that the CPU usage totals show 0% idle time:

Cpu(s): 16.0%us, 79.7%sy,  0.0%ni,  0.0%id,  0.0%wa,  1.3%hi,  3.0%si,  0.0%st

Yet when adding up the percentages in the %CPU column I get 25.9%, not 100%!