Water doesn't disappear as it's suposed to do

I was crating some water currents foe moving mobs and i notice that the water doesn't disappear as it's supsosed to do when you isolate it from the source blocks with signs. It only happend with signs. The remaining water doesn't disappear althougt you delete original source blocks.

What I expected to happen was...:
Stop water flowing by placing signs.

What actually happened was...:
The water stay there as if the current was still there.

Steps to Reproduce:
1. Create a 13x2 box where you cant put water.
2. Put source blocks at each end. (As seen in the first picture)
3. Put signs in the 6th block counting in every side. (As seen in picture 2)
4. You will get a strange water gitch. (picture 3)

PS: Apologies my English