Why am I getting worse at CS-ing by practicing it?

The first time I wanted to see how many CS I can get in practice tool without an enemy, I went 97 at 10:00 (the only reason I didn't hit 100 is that I didn't know that I wasn't supposed to push the last wave under tower, a 7 minion wave just arrives at 9:50, and I was too pushed in and my minions were focusing the tower instead of the enemy minions, so I only killed half of the wave before the timer hit 10:00). Then I got worse and worse, and went down to 84 after practicing it like 6 times. Normally in normal games I can hit 10-12 minions from the first 2 waves, but now every time I practice, I can't even get 6 out of the first 12 minions. Why does practice only make me so much worse?
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