Thank you for all your kind replies. Highly appreciated!
My bad! I thought I was using all freshly charged eneloops
and I was. So I was just doing a few pop tests with a flash meter
to compare the light output between 580exii and 600ex-rt.
What I didn't realize was that these pop tests sucked up some juice
from the 600 batteries while the 580 batteries remained "more fresh".
And hence the weird results. As Agierke had thankfully mentioned, u have got to be
using the same set of AA batteries to do the comparison.
i.e. put the 4 AA's inside 600 and then pop test; after that take out the same
4 AA from 600 and put that into 580 and pop test again. To be sure, one
should take out the same batteries from 580 and put them back into 600 and
pop test again to confirm the 1st reading. This will hopefully ensure the batteries
haven't discharged much during the pop tests.
BTW, fyi, the average recycling time on 600ex-rt was 2.44Sec on 4 freshly charged eneloop AA 2000mAH.
While the 580exii was 2.75Sec on the same set of battery.
Thank you all for your inputs once again!