Execute + Testforblock relative to the player do not work with the y coordinate

Run a super fast clock (with command blocks).
Place a CB nest to it and type : "execute @a ~ ~3 ~ /testforblock @a ~ ~1 ~ minecraft:wool"
Place a comparator to get an output and then a CB to indicate if you get an output (like "say Hi").
Go underneath a layer of wool : nothing happens !
The CB does not detect the wool block.