Why was Pluto’s discovery not actually predicted by disturbances in Uranus’s orbit?

Pluto’s discovery was not actually predicted by disturbances in Uranus’s orbit because the perceived orbital anomalies that led to the search for a "Planet X" were based on inaccurate measurements of Neptune’s mass.1

In 1915, astronomer Percival Lowell predicted the existence of a ninth planet to account for observed perturbations in the orbit of Uranus that remained after accounting for the gravitational influence of Neptune. When Clyde Tombaugh eventually discovered Pluto in 1930, it was found to be far too small to exert the gravitational force necessary to cause those specific orbital deviations. Consequently, the "disturbances" that motivated the search were ultimately determined to be the result of errors in the calculated mass of Neptune rather than the presence of an additional planet.1