They generally give lower end estimates.

If you followed along the JWST launch, it was often said that it went so well that it extended the length of the mission, but ehat happened is that the Ariane rocket performed perfectly within instead of its expected capabilities and no fuel had to be wasted on correcting mistakes, leaving more fuel for the satellite to make the adjustments required to remain fully operational at L2.

NASA suffers from a lot of restrictions, and failures are valued a lot more than successes, so rather than claiming the JWST can operate for 20 years, having something happen to cut that number down to 15 years and dooming the agency after a huge project underdelivered, they work with lower expectations.