Github compare gives different results than GraphQL output

I am trying to compare two repositories (one is fork of another), the official way, using compare and going back to 2017-02-11:

https://github.com/bitcoin/bitcoin/compare/master@{2017-02-11}...UnitedBitcoin:master@{2017-02-11}

Which returns:

There isn’t anything to compare.

But when I use Github GraphQL to retrieve all commits from both repositories, and then measure the intersection I get 218 commits that share the same sha. Below the query I use to retrieve commits from one repo:

{
      repository(owner: "bitcoin", name: "bitcoin") {
        defaultBranchRef {
          target {
            ... on Commit {
              history(first: 100, since: "2017-02-11T00:00:00Z") {
                totalCount
                edges {
                  node {
                    committedDate
                    oid
                  }
                }
                pageInfo {
                      startCursor
                      endCursor
                      hasNextPage
                }
              }
            }
          }
        }
      }
    }   

How can this explained? Why are both results different?

FYI: With "measuring intersection" I mean that I compare the IDs (sha) of each commit across both repositories.