The NETStandard.Library used is implicitly defined by the target framework. For netstandard1.3, NETStandard.Library is 1.6.1. You cannot change the NETStandard.Libary version through NuGet, but rather have to edit the csproj to manually specify it:
<!-- The NetStardardImplicitPackageVersion is implicitly set to 1.6.1 for the TargetFramework of netstandard1.3
We're setting it to 2.0.1 due to MathNet.Numerics NuGet dependency of NETStandard.Library (>= 2.0.1) -->
When you set this in the csproj, you can no longer rely on VS setting the library version if you choose to update the target framework. This, of course, is less than desirable.
Also check out the following link. There is a decent discussion there about this topic.