There are several reasons for ppd to vary by a considerable amount at any one time.
As Kougar mentioned the passkey early return bonus can boost ppd quite a bit depending the individual work unit and the speed of your hardware. You must complete ten work units with each instance of the client before the bonuses are awarded.
Any use of the computer for anything other than folding will throttle the client. In some cases the client production can be reduced to a crawl.
Each individual work unit will have a different ppd production. Within a given work unit core type the variation is usually fairly small. Variation between different core types (core 15 vrs core 17 for example) can vary considerably. Also GPU work units generally produce more ppd than SMP work units. My son folds with a 660ti and on 29 April he folded five work units for over 67,000 points, but on 30 April he folded five work units for just over 20,000 points.
I have yet to run any form of the version 7 client and was unaware that one could configure them by selecting a type of project. If maximum ppd is you ultimate goal (and who doesn't like lots of ppd) you need to be running a GPU client on both of your computers with an SMP client configured to run only three CPU cores on your i5-2400 system.
The folders who post pie-in-the-sky ppd are most likely over clocking their hardware to the ragged edge of stability and may quite possibly be using exotic cooling.
A list of the current active work units with lots of information including base points value - http://fah-web.stanford.edu/new/psummary.html