It's not odd - that's how the curation system works. You will only have a high return if you vote very early on a post that will get a large payout value in the future. So for front running the bots, you need to both have a post that has a lot of outstanding bids AND has a very low current pending payout, which is not very common.

It does happen every once in a while though, so it's worth checking here and there to see if you can find a diamond in the rough.

Also that I should note that unlike paying to promote your posts, you're just using a vote in this case, so even with a negative ROI you're not actually losing any money. Now that i'm thinking about it maybe I should change it from showing ROI since that's maybe not the right way to look at this in terms of vote value.