Without doing any extensive research --
I think the "lowest" "economy" fare is actually (as you are seeing) CLE-IAH (C) + IAH-GRU (Y). Whereas the Business fare is properly a through-fare for CLE-GRU (D). It appears that CLE-IAH is pretty close to sold out in economy that day at that time, which forces combination with an expensive IAH-GRU economy fare.