For first question- Nuget package has different builds for different framework such 4.8,6,7 etc.. so when we reinstall library even though version is same, reinstall tell Nuget to pick new target framework e.g lib/.netstandardlibray/mylibrary.dll

for second part - some library still point to older folder location instead of newer is may be due compatibility fall back. That is only version which is most compatible to newer framework.