Man has about 21,000 genes, a mouse has about 23,000 and a water flea has 31,000 genes the most of any animal. How can this be considering man has so much more than a flea?

Man has about 21,000 genes, a mouse has about 23,000 and a water flea has 31,000 genes the most of any animal. How can this be considering man has so much more than a flea?