This doesn't make much sense, Webb was deliberately underboosted for L2 orbit because it has thruster only on one side and cannot correct for any overboost [0], so if Ariane performed as expected there should be exact calculated amount of fuel left. If there would be more fuel left in Webb than predicted it would actually mean that Ariane overboosted from programmed trajectory, which is bad thing. I think this is just NASA doing its usual underpromise-overdeliver rather than better-than-expected performance from launch vehicle.
[0] (supposedly it cannot rotate its cold side to the Sun, though I don't know what difference it makes when it's not taking images at the time?)
[0] (supposedly it cannot rotate its cold side to the Sun, though I don't know what difference it makes when it's not taking images at the time?)