Issue in what if parameter

I set the what if parameter to range from 1 to 99999, with increment of 1.  Because of the way that I am using the what if parameter, I need the user to input a specific number, and not just use the slider.  

The problem is that sometimes, only sometimes, the number changes after I put it in. Why is this happening??? So for example, if I put in 3080, it automatically becomes 3079. Sometimes the change is from say from 4151 to 4149.