Why are there Microsoft packages in my private Nuget Feed in DevOps Artifacts?

Here is my build pipeline

# .NET Desktop
# Build and run tests for .NET Desktop or Windows classic desktop solutions.
# Add steps that publish symbols, save build artifacts, and more:
# https://docs.microsoft.com/azure/devops/pipelines/apps/windows/dot-net

trigger:
- master

pool:
  vmImage: 'VS2017-Win2016'

variables:
  solution: '**/*.sln'
  buildPlatform: 'Any CPU'
  buildConfiguration: 'Release'
  Major: '2'
  Minor: '0'
  Patch: '0'

steps:
- task: NuGetToolInstaller@0

- task: NuGetCommand@2
  inputs:
    restoreSolution: '$(solution)'

- task: VSBuild@1
  inputs:
    solution: '$(solution)'
    platform: '$(buildPlatform)'
    configuration: '$(buildConfiguration)'

- task: VSTest@2
  inputs:
    platform: '$(buildPlatform)'
    configuration: '$(buildConfiguration)'

- task: NuGetCommand@2
  inputs:
    command: pack
    packagesToPack: '**/*.csproj'
    versioningScheme: byPrereleaseNumber
    majorVersion: '$(Major)'
    minorVersion: '$(Minor)'
    patchVersion: '$(Patch)'

- task: NuGetCommand@2
  displayName: 'NuGet push'
  inputs:
    command: push
    publishVstsFeed: 'MyCommonFeed'
    allowPackageConflicts: true

I was surprised to see that the feed created contains packages from Nuget.org Why?