Well I'm not too surprised because caffeine produces cerebral vasoconstriction, which actually improves blood flow because the heart can send more blood when arteries are constricted (higher blood pressure). Caffeine WD of course causes the opposite, cerebral vasodilation, which appears to be responsible for the headaches.
And it turns out amphetamines and other stims also cause vasoconstriction (mostly via a different mechanism), so that might compensate for the vasodilation caused by caffeine WD.