Why is the border width not the same as what I set in html?

I set a border style of a div element in html as \[code\]border: 1px solid black;\[/code\]. It shows correctly in most cases. But I just found a weird one, which is Chrome on Win 7 Home Premium, that the computed style shows the border width is 1.11111111px instead of 1px. Could someone tell me any possible reason that makes this happen?
 
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