By design according to Frogboy, ogres have extra essence around them, perhaps it is something in the lore.

I admit that after playing around with TileYields quite a bit, it's unclear to me what makes them show up on the map, and I can't say that they do or don't always work. For example, I attempted to make cliff/beach tiles have yields, and that didn't do a thing (visually, or on mouseover). And yes, I know that there is a setting you have to set in the environment section to make the yield go active...

The yields you see on tiles in game are not the yields of that one tile, they are the cumulative yields of the surrounding tiles. It's sort of like CIV where a city uses multiple tiles around it, except simplified so you only have one number to look at. You are never going to see a yield number on a tile you can't build a city on, but the yields of the tiles surrounding it will be boosted by whatever amount.

OK so as it turns out it does touch on this (briefly) in the manual.  Thought i'd post the paragraph here for completeness, underlined bits are my emphasis.