It's an indication or a range, not a surefire thing. The human body is hard to control, so no doctor is able to predict in advance how long an injury takes to heal. This is the same in real life, some injuries just linger.
At least in Buzzerbeater you know for sure that if an injury has a 1 week indication, it won't take longer than 13 days to heal (then it would be 2 weeks).