Answer
The number of duplicate genes drops due to loss of whole chromosomes or loss of parts of chromosomes. This occurs at different rates in the multiple genomes of a polyploid.
Work Step by Step
It is not completely clear why this happens, but it may be that it has something to do with replication rates for DNA. Also, if there are duplicate genes, some copies could be lost without losing gene function, leading to a fully functional but more efficient organism.
