Why do multiple and lethal alleles often result in modifications of the classic mendelian monohybrid and dihybrid ratios?

Multiple alleles can modify the classical Mendelian ratio by increasing the number of phenotypes.  

For example, in a monohybrid cross, Mendel gets only 2 phenotypes however, in ABO blood groups (a classical example of multiple alleles) there are four phenotypes present in a population.

Lethal alleles are the alleles which when expressed result in the death of an organism. These alleles can be dominant, recessive or conditional (depends on interaction with another gene).  

Thus, lethal allele reduces the ratio of the phenotype associated with it by affecting the survival of the organism.

For example, lethal yellow allele in mice results changes the phenotype ratio from 3:1 to 2:1