It’s an estimate only. The human femur actually provides a fair amount of information. They can tell if you’re male or female depending on the angle of the femur head. They can tell what your musculature was like because of how thick the muscle attachments to the bone. Recently, they’ve figured out they can determine how much you weighed by how thick the femur is (makes sense, as the femur is weight bearing), but it can’t tell if that weight is muscle, fat, or even pregnancy. The epiphysis can tell them if you’ve reached adulthood, and wear and tear on the joint ends can allow them to estimate how old/how active you were. I don’t know if there are any racial markers available on the femur.
However, when it comes to your height, they’re working off a table of averages, almost like insurance agents. So, if you’re an outlier - very long legs for your body type, they’re going to come up with a height that’s actually above yours. If they only had your legbone to work from, that would be that. If they had other bones - pelvis, vertebrae, skull, they might be able to pick up enough information to see that your legs were long in proportion to the rest of you and adjust for that. The skull, of course, would give them a great deal more information about possible race and age.