In a monohybrid cross, why is the F2 phenotypic ratio 3:1 but genotypic ratio 1:2:1?

Yes, they count the same offspring, but phenotype is the visible trait while genotype is the actual gene combination. Take a cross between two heterozygous tall plants, Tt and Tt. The F2 offspring are TT, Tt, Tt and tt, giving the genotype ratio 1 TT : 2 Tt : 1 tt, which is 1:2:1. Now look at appearance. TT is tall, both Tt are tall because T (tall) is dominant and masks t (short), and only tt is short. So three plants look tall and one looks short, giving the phenotype ratio 3:1. The difference exists because the two Tt plants and the one TT plant all look identical (tall) even though their genotypes differ. So one visible class can include more than one genotype.