@Luvfishin - What project(s) are you running? Like the others above have mentioned, depending on the projects, run time per result can vary quite a bit. The mac pro may be getting more of Project A, which has shorter run times, while the iMac is getting more of Project B, which has longer run times.

If both machines are set exclusively to one single project, then the only explanation I would be able to think of is that the iMac is consistently getting bigger jobs to run. Some work has larger jobs, then smaller sub jobs to run. I don't know how best to describe it. But the larger job may take several hours to run, while the smaller job may be done in an hour or less.

Either way, it looks like both your systems you listed are producing the amount of work that should be produced, at least by runtime. If you are hitting thermal throttling on either machine, that may play a role. traditionally laptops and all in one systems have less cooling due to smaller footprints to work with. So if the systems are getting too warm, try getting better ventilation around them. i.e. laptops can be on a cooling pad, make sure the all in ones have plenty of room around them to "breathe" in the cool air and exhaust the hot.

I am running single cancer projects which take up to eight hours exclusively. The Mac Pro is running all of the projects and I'm sure that it is getting many smaller jobs, thus giving it more completed units.

As per heat, all of my iMacs are in wide open spaces and not cluttered. I should monitor the heat more closely though.

I am no Mac user, so I can't help there...
But googling "os x cpu temperature" seems to return useful results...

Thanks, I Just Did a Quick Test Using istat. Temps Were between 70 C-74 C.

What Are Your Temperatures?