Because in this case you consider your release to be 15.1.2 which is within the vulnerable range. Quoting from https://my.f5.com/manage/s/article/K51812227:
Versions known to be vulnerable: The range of product versions within each branch that are confirmed by F5 Product Development as vulnerable. Point releases and hotfixes are not listed in this column, unless a vulnerability is specifically introduced in a given point release or hotfix. Vulnerable versions include all point releases or hotfixes for a given software version. For example, if 13.1.0 is listed as vulnerable, then 13.1.0.1 and 13.1.0.2 are also considered vulnerable if neither of those point releases are listed in the Fixes introduced in column.