Is there a difference in runtimes between the work units? A recent 4-core unit took longer than a 3-core unit!

I no longer run n-body. They have had some problems in the past.

What was the difference in time?

You might want to run a resource monitor to see how many cores it is using.

I have a tool here that I used to do a credit comparison on your 3 valid results. Ideally for the same amount of credit the cpu time should be the same

        Run Time     CPU Time     Credit
         (sec)         (sec)
            11.6          43.1          1.0
            26.1          74.6          1.0
            21.2          59.5          1.0

normalizing can show problems with credit calculations
or which gpu devices are faster or slower than others

Looks like the top task had fewer CPUs available than the bottom two