Your only specified from April 25 to May 5 as your contiguous range of snapshots to “destroy”. Hence, it is telling you this will only liberate 14 GB.
If you try a larger range (that goes back even further), you might find it liberates much, much more space.
Imagine a truck with boxes. The load is getting full and you need to “free space” by removing some boxes.
Newly added boxes are always tagged with a white sticker. In order to physically remove a box from the truck, you must first remove its white sticker.
However, in the past, someone wanted to “protect” some boxes, so they slapped blue stickers on many boxes. Another person did the same thing, but with red stickers. And another person did the same thing, but with green stickers.
If a box has even a single sticker on it (of any color), it cannot be physically removed from the truck.
You come along and remove the white stickers from all the boxes you want taken out of the truck. But you cannot yet physically remove the boxes, since they still have at least one sticker on them.
Someone comes along and asks “How many boxes can we free up if we remove all the blue stickers?”
You answer: “Not much. Almost none.”
Someone else comes along and asks “How many boxes can we free up if we remove all the red stickers?”
You answer: “Not much. Almost none.”
Someone else comes along and asks “How many boxes can we free up if we remove all the green stickers?”
You answer: “Not much. Almost none.”
Finally, someone different comes along and asks “How many boxes can we free up if we remove all the blue, red, and green stickers?”
You answer: “SOOO MANY BOXES CAN NOW BE PHYSICALLY REMOVED!”
See how that works? Only looking at one sticker color at a time doesn’t really tell you much, since there are multiple colored stickers involved.
Now replace “boxes” with “data blocks” and “colored stickers” with “snapshots”.
EDIT: See this post that has pretty pictures for a better explanation.