It's possible that the duration you get is still just down to individual variability; maybe you metabolise it particularly quickly. All the same though, your starting dose was 90mg!? That's practically the maximum dose for most people. Was it 90mg XR, or 30mg 3x/day?
I guess I could try to argue that the duration of effects you've been having since the beginning is purely just down to your starting dose, but methylphenidate seems to be so ridiculously hit-and-miss from person to person (for some it's like cocaine, for others, like me, it's pure anxiety in a pill) that I honestly can't say I'm all that surprised.