The answer to your question has been posted 3 times in the last 50 posts. But to sum it up for you, GPU has overhead loaded into it. Either put a 6 or 8GB card as GPU0 or use on-cpu video.
I get the problem even when disabling the first GPU. The overhead is a poor excuse for a miner that is simply outdated. His Zcash miner is even more outdated.