Ariane performed remarkably well, and that fact should be recognized and praised in the media. It's not exactly wise to hope for "errors in your favor" - much better to be more-precise-than-expected, as it allows them to plan more daring maneuvers with less margin in the future :)
JWST has thrusters on only one side, and to correct for an excess of velocity it would have had to rotate such that its instruments faced the sun, ruining them instantly.
As such, the mission design called for a deliberate underboost, to be made up for by JWST's own precise thrusters. But a more precise launch vehicle means that the error bars are smaller, and they can use a much smaller underboost than would be required from a less precise launch vehicle.