Sample Operator - Probability

I have to admit, I am having a hard time understanding the output from a Sample Operator when selecting probability as the sample parameter.  

For example, if I use Generate Data to create a 100 example ExampleSet, and I connect the Sample Operator with probability and .1, I get 7 records.  

In short, why is it not 10 records?   I am having a hard time wrapping my head around this.