Is there an intuitive reason why Hillary Clinton won the popular vote amongst people voting for President, yet the GOP won the popular vote amongst people voting for their Representative in the House? Is the answer 'lots of vote-splitting by people who expected Clinton to win and wanted a GOP House as a check on her power'?
President: 63m for GOP, 65.9m for Democrats (Wikipedia article on 2016 presidential election results)
House: 63.2m for GOP, 61.8m for Democrats (Wikipedia article on US House of Representatives elections, 2016)
- But see also: 'Republicans captured the majority of the "popular vote" for the House on Election Day, collecting about 56.3 million votes while Democrats got about 53.2 million, according to USA TODAY calculations.' (Incorrect? USA Today article)
- Wikipedia 2016 House Vote Summary, shows Republicans gaining 49.1% of House votes compared to Democrats at 48.0%
Note that comparing total votes cast, it seems an additional 4 million votes were cast for the presidential election compared to the House elections. This makes answers that just reference vote-splitting incomplete, to my eye at least...
EDIT: Since posting this question, I wanted to draw readers' attention to a pretty striking map that possibly explains part of the answer: Source: Ballotpedia Hat tip: @lazarusL
It seems that there were more no-Democratic House districts in 2016 than no-GOP House districts. In these districts, Democratic voters would have voted for Hillary but would have been mechanically unable to vote for a Democratic House candidate.
Edit #2. Ballotpedia's margin of victory data for all 435 House races (see https://ballotpedia.org/United_States_House_of_Representatives_elections,_2016) lists 12 CRs with GOP unopposed and only 5 Congressional districts with Democrats unopposed. Let's say avg 250k per district, and perhaps 25% of them vote for the presidential candidate from the party that hasn't fielded a Congressional candidate. This leads to a rough estimate of half a million (net) votes to HRC that didn't go to a Dem Congressional candidate. So although the unopposed races story was really elegant, perhaps the bigger story is in fact vote splitting.
Edit #3: Ballotpedia list of unopposed districts doesn't count some districts where a candidate ran against another candidate from the same party, or from a third party. However, these omissions are if anything skewed towards Democrat House candidates running up the vote tally (see Brythan's answer), which makes it even less likely that the unopposed races story is a key component of the answer here.