Different symptoms of ACL tear?

When my mom was my age she went skiing for the first time and fell repeatedly. Finally a fall was bad enough that she blew out her knee. She went to the ER but they didn't diagnose her with anything and never returned to the DR. In May of 2009 I wrecked my dirt bike and heard a loud pop and believed my ACL to be torn. But because my knee wasn't as swollen as my mom's had been and because I had no bruising my parents believed me to be okay and I never went to the doctor. A month and a half ago I finally had an MRI and it turns out I had a full ACL and medial mensicus tear and I have gone through re-constructive surgery. Out of curiosity my mom also had her knee checked and her ACL has been torn for almost 26 years. Why is it that our symptoms were so different when our injuries were the same? Why would her knee have been black and blue while mine only swelled? Was it due to the means of injury, that mine was fast and occurred within seconds while her's was from repeatedly twisting the joint from falling until the stress was too much? Thanks! Just curious!