According to Astrophysicist Becky, the JWT launch was so close to target that the useful life can more than double. She is a black hole specialist and makes any dry subject like that entertaining - don't just trust me - trust yer own eyes (YouTube & the JWT launch)

This is because the fuel needed to get to the Lagrange point and the accuracy thereof used the minimum budget and left more to keep the instrument on top of the (notional) hill top. The Earth's, the Moon's and the planets have less than perfect trajectories - mostly due to the gas giants like Jupiter's mass modifying local gravity. In short, it ain't exactly a three body problem - in reality.

FWIW I found this guy's explanation of Lagrange points simplistic enough and entertaining (given the techie nature) for it to stick in my head - is that a Lagrange point for Lagrange points?