At the November 6 election, voters in the 31st U.S. House district cast 207,095 votes for President. But those same voters only cast 161,219 votes for U.S. House, which means that 22.2% of the voters in that district chose to abstain from voting for U.S. House. This is undoubtedly because the ballot only listed two Republicans for U.S. House, and did not permit write-ins. So, many voters in that district disliked both candidates, and therefore abstained.
There are always some voters who cast a vote for President and then fail to vote for U.S. House. For example, in 2008, in California, 8.6% of the voters who voted for President did not vote for U.S. House. Obviously, though, there is a significant difference between 22.2% and 8.6%.
The vote in November 2012 for President inside the 31st district was: Obama 118,043; Romney 83,822; Johnson 2,027; Barr 822; Stein 807; Hoefling 721; various write-ins for President 853. The vote for U.S. House for the two Republicans was: Gary Miller 88,964; Bob Dutton 72,255. The 31st district is entirely in San Bernardino County.