It looks like Zwift are using the same old bassackwards system for this mission as was used in the Wahoo Climbing Mission of January this year, and that was criticized at the time.
Basically, the game takes the user’s total vertical gains, calculates that value as a percentage of the total (2500 metres), rounds that number to the nearest whole number (up or down), and then generates the total elevation in metres from that rounded value. Thus, the actual value indicated in metres could be wrong by +/- 25 metres, just so that the numbers displayed in the mission tile in game are whole.
Since we see the totals in metres gained at the end of each ride, it would be more accurate for the value in metres to be correct and for the percentage of mission accomplished to be displayed to more than zero decimal places, but I guess it doesn’t look as clean. 