It is because if the dwarf is the dominant character but is not having the same allele and it is having heterozygous form of allele .as the generation is is the the dominant but after two generations in the third generation the recessive character is seen that is all the allele are same and having the recessive character allele.
Even though dwarfism is a dominant gene, it does not occur frequently because if the dominant allele in F1 generation is dwarfism then the offspring will be tall. in other case if the offspring gets both TT alleles then also the dwarfism becomes recessive. so most of them are not dwarfs.