> there should be exact calculated amount of fuel left
Nothing is exact :) Ariane upper stage boosts JWST on a certain trajectory +/- some error in all directions, and then detaches. Along the way, they figure out the error and JWST corrects it with trajectory correction burns. This is somewhat independent of any fuel left for the "underboosting" you're talking about.
A mission designer looks at the performance specs/error distribution for Ariane and tries to include enough fuel so that, 99.X% of the time there will be enough for trajectory correction maneuvers AND a 10-year lifetime. When the (Ariane) burn is precise, errors are small, and less fuel is wasted on TCM, leaving more for the mission itself.
What this (10->20 year lifetime adjustment) really means is that NASA's left quite a lot of margin for TCM here... Which, for something this expensive, was a good thing :)
Nothing is exact :) Ariane upper stage boosts JWST on a certain trajectory +/- some error in all directions, and then detaches. Along the way, they figure out the error and JWST corrects it with trajectory correction burns. This is somewhat independent of any fuel left for the "underboosting" you're talking about.
A mission designer looks at the performance specs/error distribution for Ariane and tries to include enough fuel so that, 99.X% of the time there will be enough for trajectory correction maneuvers AND a 10-year lifetime. When the (Ariane) burn is precise, errors are small, and less fuel is wasted on TCM, leaving more for the mission itself.
What this (10->20 year lifetime adjustment) really means is that NASA's left quite a lot of margin for TCM here... Which, for something this expensive, was a good thing :)