Why are metallic hydrides written as LaH _2.87 , YbH _2.55 , PdH _{0.6− 0.8} ?

The number represents the ration of the Hydrogen with the metal atoms.

For example..

LaH _2.87  means the ration of the number of La atom to the number of H atoms is 1:2.87. This is due to the defect in the crystals that the compound are non stoichiometric.