In short: The First Past the Post Voting system.
The Ulster Unionists will get that many votes but in a small number of seats in Northern Ireland. The Conservatives might just win by a small amount but in a large number of seats, each of which translates directly to an MP in Parliament.
For anyone who hasn't watched them , I can wholeheartedly recommend CGP Grey's "Politics in the Animal Kingdom"[0] videos for an brief overview of why FPTP voting is as bad as it seems.