Why does the DNA of Bacillus subtilis not contain DNA families of 20-37 members?
Based on the paper (open access) from which this figure was adapted, the 38-77 group consists of 5 gene families with 38, 47, 57, 64 or 77 members. In contrast, there are 284 “families” with 2 members, 91 with 3 members, 42 with 4 members, 25 with 5 members, etc. This is clearer in the original figure:

Thus the larger gene families are outliers and there simply exists no gene families in B. subtilis with 20-37 genes (at least by the methodology of this paper).