Something is strange in the math for "Let's stretch your time off from X days to Y days"

I am in the US, and it says I have 11 days off. If I say I get 0 days off, it says: "Let's stretch your time off from 0 days to 18 days". It looks like it's counting the weekends prior to a holiday no matter what as a "bonus" day, which is a strange methodology to me. (I would expect that it would only count days that it performed some action to optimize, otherwise it's just table stakes)