As he said, it's usually the amount of self vote. The vests make the same value of vote regardless if you cast the vote or if @randowhale does it.
2 key factors that change that but both mitigate each other:
His VP gets drained from many votes which would shrink the vote value and thus your profit overall
Between the amount of votes he does cast, he's more likely to catch blogs prior to rapid inflation which would increase the curation rewards given to you