How to detect Single Nucleotide Variants (SNVs)?
Your link doesn't link to the paper, and you seem to be ignoring half of what the image is trying to get across.

The point is that since tumors are heterogenous, you very well might have only 40% of the cells heterozygous for a particular SNV, which means it would only be observable in 20% of the reads.  But if multiple reads with different start sites show the same SNV, it's likely to be real.  If coverage is so low that you only get one read showing a real SNV, you won't be able to distinguish it from noise.