Off topic:What'S "Allelic Ratio"? Can It Equal 1 In Normal Cells?

Question

[Cheng Guo]

Hi, I'm a beginner in analyzing NGS data. This is my first time reading a paper. However I'm confused about the concept "Allelic ratio"

My question is: why the y-values (Allelic ratio) in the figure below are all between 0.2 and 0.8. Can "Allelic ratio" equal 1 in normal cells?
Figure legend: "Allelic ratio data of heterozygous loci in the normal genome are centered around 0.5, which represents the presence of two alleles". It's figure 1a in this paper.

Here is how I think about "Allelic ratio":

So, I think the ratio should be about 1 for a SNP genotype AA; 0.5 for AB; 0 for BB. But few points in this figure have a ratio more than 0.8 or less than 0.2, I'm confused about it.

Thank you for your help.