I've wondered about these myself.
It's not "half the water source block", it's the full water source block, it's just the way the water moulds itself to its surroundings. For instance, make a 3x3 dirt pad, except for the middle block put water. Observe the water level. Now remove one of the corner blocks (so that the water is still contained, just a caddy-corner block is missing). Observe that the water level in that corner has lowered. Repeat with the remaining 3 corners and observe that the water level appears to have lowered to about the same as these "Springs". It's still a normal water source block, and if you grab it with a bucket of water it'll still behave normally.
Similarly, I believe if you allow a block of ice to melt naturally (light) without any block updates, it will do the same thing.
Anybody know what mod adds these "Springs"?