How can two members of the same kingdom belong to different species, while two members of the same species must belong to the same kingdom?

The biological system of classification is based on the following ranks: (From highest to lowest)

Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species

According to this ranking, the organisms with more common generalized characteristics will belong to the same kingdom. The organisms which will have more specific characteristics in common will belong to the same species.

Hence, two members of the same kingdom may or may not belong to different species but two members of the same species must belong to the same kingdom.

For example, reptiles and mammals will belong to the same kingdom animalia but they but they will not have similar species.