Animals are either Heterozygous, Homozygous or neither for any given mutation.

When a Homozygous animal is bred to a non-gene carrier all the offspring are Heterozygous (100%).
A Het X Homozygous produced half a litter of Homozygous and half a litter of Heterozygous animals (100% hets).

The 66% possible hets and 50% possible hets denote the chance those animals statistically have of being heterozygous.

A Het X Het pairing will produce 1/4 Homozygous animals. The remaining 3/4 of the litter are 2/3 hets and 1/3 non-carriers. Because 2/3 of those are hets, but you can't tell which ones are, we say they have a 66.6% Chance of being hets, aka they are 66% possible hets.

Breeding a Het X non-carrier produces statistically 1/2 hets and 1/2 non-carriers. Because you can not differentiate between the two we say that the offspring have a 50% possible chance of being hets.

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