Is this a bug, why does this happen? Level difference

I think I was vs Teemo, and I got first blood and had around 27 cs and Teemo had 20 but the problem is that he was ahead of me by 1 level, I was wondering how? How could this be possible? After this he got an assist on jungle, and was 0/2/1 while me 2/1/0 but he still was ahead by 1 level while I had more cs? How come? This also happens vs Pantheon?
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