Mainly it depends on the moment of the payout. Example:
User A and B delegated the same amount.
User A receives payout.
User C buys upvote for 30 SBD.
User B receives payput with the 30 SBD payment already included.
User A will receive the the profit from 30 SBD in the next payout (it was added to his pending payout).

Also the curation reward is added to STEEM payout after 7 days from the start of delegation (because curation rewards are paid with post payouts after 7 days).