they likely have the real polls themselves and determine the buffer they need to pass. Thing was, polls aren't 100% and maybe their data was skewed so the did say a 2% swing when really they needed 3%.That's why the fake polls they broadcast always show a close race so it's not obvious when every election one person just barely beats the other. This time they corrected their algorithm and had to do 6% to guarantee the victory. Problem is, trump got even more votes than last time and so the swing become overwhelmingly obvious.