Apart from “lower PIUS”, I believe I understand your questions, I just don’t have the answers.
Obviously the boards are different, but that doesn’t mean that the drives are necessarily different internally. The dissimilar boards wouldn’t explain the significant difference in power consumption, though, assuming the ratings are correct.
In any case I don’t believe it is a FALS drive. AFAICT, the FALS drives have a different PCB and MCU chip.
BTW, the power specs for the WD1002FAEX and WD1001FALS drives are identical:
WD Caviar Black Series Disti Spec Sheet:
As for the interface speed, try WDSSPEED and see what happens. If the MCUs are identical, then the only reason for the speed difference will be the firmware. Quite often a manufacturer will produce essentially one product and then cripple its firmware to artificially generate a range of lesser performing products for the lower end of the market. I’m not saying that’s what’s happened in this case, though.
If you would like to pursue your suspicions, show us the component side of each board. Then we will be able to see the MCUs, and we could also compare the date codes on the chips against the date codes on the labels.