For some reason Atmel decided that it would be OK to require higher VCC for some programming functions on the WD device. 6.5V is required on VCC for chip erase and lock bit writing.

Now we can't use the WD as a drop-in replacement on existing in-system programming designs, because we tied VCC directly to the 5V plane. On the original AT89C55 all that was needed was 5V on VCC, and 12V on EA.