Primary residence stay "2yrs<1yrs" changes return?

[Sean H.]

I was doing turbo tax, and im surprised that it is changing my return based on the number of months i claim i used the residence as my primary residence. 

I realize over two years is tax free, but im not trying to do this. 

I had stayed there for 19 months, and it claims my income is totally tax deductible (which is weird). So out of curiosity i lowered the number down a couple of  months (all the way down to 13 months), and it gave me a different number every time i changed the number. Is this a flaw in their software, are is there really a deductible based on time less than 24 months?

I also realize that over a year is long term capital gains, but that is not my question either.