Here's the link to Dr. Allen's post on the matter:
The only part I'm not clear on is how they determine the credit per CPU second.
Essentially it works out that roughly 1GHz class hosts and faster will get long WUs.
Alinator
Hosts whose benchmarks place them among the slowest 20% of hosts are given short WU if possible. The remaning 80% of machines get both slow and fast WU.
Cheers,
Bruce

slow work
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Your machine must have microscopic benchmark values. What happens if you use the BOINC manager to re-run the benchmarks when the machine is idle? Do the benchmark values change? Note: you can find the benchmark values in client_state.xml
Bruce