Drug test results and the cut-off levels the results are based on don't reflect levels at which a person would be impaired; purely because this is virtually impossible to determine.
Of course, extremely high levels are likely to reflect impairment and at extremely low levels are unlikely to reflect any impairment at all. However the point at which people actually become impaired varies immensely.
This is similar to alcohol consumption where people who rarely consume alcohol (or who are very slightly built) tend to become inebriated much faster than those who have previously had a long history of alcohol consumption or have larger bodies. This is the case even if both people have consumed exactly the same amount of alcohol.
Most people would agree that anyone with no alcohol in their system is unlikely to be impaired and those with very high levels of alcohol in their system are very likely to be quite impaired. Once again - the point at which people actually become impaired from alcohol varies immensely and this is fairly widely understood.
However there is of course a stark exception to this where low or 0 levels doesn't mean that the person isn't impaired. Anyone who has, consumed a lot of alcohol will be well aware that a hang-over can last an extremely long period of time. In fact this could even be well past the time that the person has returned to a blood alcohol level of 0.000% BAC.
A person can still feel fairly "crappy" for a long period of time and not perform at a 100% level of response and precision (usual measures of impairment) long after any trace of alcohol has left their body.
Using this same analogy; a person who has for example consumed a drug like methamphetamine might be extremely fatigued while coming down off the drug well past the point at which the person returns a negative test result.
Another stark exception is the issue of tolerance. As with alcohol, medications or any other drug - people who consume large amounts frequently can build a tolerance and display few obvious signs of impairment even at higher levels. In fact greater tolerance can mean that high volume frequent users can become impaired without any trace of the drug at all (withdrawal).
More frequent consumption of a drug usually reflects an upward shift in the impairment "window". In other words greater consumption leads to greater tolerance which in turn requires greater levels of consumption to achieve/ reflect impairment. A new window of impairment also appears at low levels (the withdrawal described above)