posible scoreboard problem or i being stubid

I Had Problem to get redstone working and i played a round little bit and i found this... If i use command /testfor @a[playerKillCount_min=15] then i added some points first try it worked well but when i set my score back to 0
/testfor commandblock still finds me and sends a rsignal

What I expected to happen was...:
Command didn't notice that i resetted my score and didn't shutdown a redstone signal

What actually happened was...:
Signal kept being positive

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. ...dont know
3. ...