a. The first explanation may be that the ancestor of both chimpanzee and humans had three copies of the gene and one was lost after the human ancestors diverged from chimpanzee.
In addition, the genes that were functional in their common ancestors are dysfunctional in humans. Pseudogenes are relatives of genes that have lost their protein coding ability in the humans. This is because of the deletion or to the accumulation of mutations. These genes are not required for the survival of the humans. These changes may contribute to differences in physiology.
Alternate one says that the common ancestor had duplication, which occurred in the chimpanzee line. Duplications of chromosome segments in a single lineage have contributed to the genome divergence. More than 170 genes in the human genome and more than 90 genes in the chimpanzee are present in large duplicated segments. These duplications are caused due to a greater amount of total genome divergence. It is not clear whether they contribute to major phenotypic differences or not.
b. We can distinguish between the two possibilities by determining number of copies that are present in the other great apes. This would allow us to determine occurrence of change in evolution of apes.