Why HMS Eagle has several full-load displacement figures

Why is it difficult to find one full-load displacement for HMS Eagle in the interwar years?

Because Eagle was repeatedly modified and used for experiments, so her fuel capacity, aircraft load, stores, and resulting full-load displacement changed over time.

A full-load displacement includes fuel, aircraft, stores, ammunition, and other variable weights rather than describing an immutable hull. Eagle changed frequently because the Royal Navy used the converted battleship as an experimental carrier and modified her arrangements from year to year. Her patched-in fuel system was unusually accessible, making the ship a convenient test bed for fuel-management changes that altered capacity. Figures for 1924 and 1925 can therefore differ legitimately even when both sources are careful and technically correct for their dates.