Cannot comment on the previous answer, but the targetFramework attribute in packages.config is the .NET version of the project at the time that package was installed.
For example, I have two projects that use Newtonsoft.Json 9.0.1, and these are the lines in their respective packages.config files:
<package id="Newtonsoft.Json" version="9.0.1" targetFramework="net452" />
and
<package id="Newtonsoft.Json" version="9.0.1" targetFramework="net462" />