When I "top" I see the following:

PID  COMMAND    %CPU TIME     #TH 
7867 perl5.10.0 96.6 26:15:19 8/6
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(where #TH: total number of threas/number of running threads)

I would interpret that to mean that at the moment in time when you ran the command, 6 of your 8 threads were running concurrently, and between them they were using 96% of your total cpu capacity.

In other words, if you have a single threaded process running flat out on a single core, I would expect the highest %cpu value shown to be 100/8 = 12.5%.

The fact that at the instance that the command ran, only 6 of 8 threads was actually using a core, just means that in that brief instance, some other higher priority processes--like your display driver, or your network driver, or your disk driver--needed to do something.