Bar weights cannot eliminate handlebar vibrations, they merely serve to shift the resonance point of the bars to some other point in the operating range. The 'vibrations' people notice are a resonance mode in the bars. You can shift where this resonance point is by either altering the mass or stiffness of the bars. Increasing the mass and decreasing the stiffness shifts the resonance point lower in the revs, and doing the opposite shifts it farther north.
You by lightening your bars are shifting the point farther north in the revs. I imagine if you ride a little faster, you'll again find the point where the bars do vibrate, just not where you ride most of the time.
It's Vibes...nothing more, nothing less.