My Brillant Daughter!

Laurie is extremely bright and I just have to brag on her. I have a program on the graphing calculator that goes over basic facts with integers. I wanted her to practice her subtraction facts, so I set it up for her to do 20 problems. I forgot that the program would give problems like '4-5=?'. When she got it wrong-she asked me about it. I explained that when the first number was smaller than the second number, she would get a negative answer. I showed her how to make a negative number on the calculator, and she continued to get the rest right. No other explanation needed-she just got it! I bragged to Jason-he said it was genetics. I'm just wondering why my 8 year old can get it, but my middle schoolers and freshman couldn't...