In Hanafi jurisprudence, the classification of witr as wajib shows that legal obligations cannot be reduced to only two categories: fard or voluntary. Performing witr is binding, though its evidentiary basis is not placed at the same level as an indisputably established fard.
The daily obligatory prayers have a different evidentiary standing regarding their prescribed units and times. Calling both acts “prayer” does not make their technical classifications identical.
Other schools express the ruling of witr differently, so this example specifically illustrates the Hanafi classification, not a universally agreed terminology.