Here's why:
* Recessive traits only appear in the phenotype (physical appearance) if an individual inherits two copies of the recessive allele.
* Dominant traits will be expressed even if an individual only inherits one copy of the dominant allele.
So, a cat with at least one copy of the dominant allele for normal tail length will have a normal tail, even if it also carries the recessive bobtail allele. Only a cat with two copies of the recessive bobtail allele will have a bobtail.