this may have been the case when the neck was made, but if you fully slacken off the truss rod any time afterwards, then it may have picked up a bow one way or the other. the idea of the notched straightedges is that you get the fingerboard straight by adjusting the truss rod & only then level the frets. if you try to level the frets without the wood straight then you'll end up grinding down the frets either in the middle or at the two ends unnecessarily & probably introduce other issues.