In order to avoid insecurity issue , the NNPC HR might find it difficult to call a large turn-out to towers on same day for and also the total funds for the whole sucessful candidates might have not be released to NNPC to release the letas at once.We all know what is happening with Min of Finance , releasing money bit by bit. When u get employment letter, wouldn't be paid. So lets be patient for the whole process be completed.