How? One factor is Washington’s Redistricting Commission packed too many Democratic voters in Seattle, while other similar voters were sprinkled around other places in the state.
For example, in the 2012 election, the Democrat in Seattle's WA07 won 298,368 votes, for 79.65 percent of the vote. This is in an election with 85 percent voter turnout!
I live in the southwest corner of the state in WA03. In the same election the Democratic candidate earned 116,438 votes for 39.62 percent of the vote.
In WA07, for the all of the turnout, Democrats got one seat. There could have been 100 percent turnout for the Democrat and the result would be the same. In WA03, for a decent showing of 40 percent—Democrats got nothing. This used to be a competitive district, however, it was the first election within the new 3rd district; where the Commission drew Olympia, with all its Democratic state employees, out of the district. This basically made the 3rd a GOP safe seat.
Single-member districts can skew outcomes through arranging voter populations. Combine the Democratic voters in the districts mentioned above and see how 414,806 votes won only a single seat. This leads to how majority Democrats lost the US House in 2012, and very well could wind up in the same place again for the 2018 election.
The way we elect US House members is the result of political decisions made in various states and US Congress. There is no mandate in the United States Constitution for 435 House seats elected with single-member districts. Here is Drutman on the Fair Representation Act. It is about using proportional representation to elect House seats as a way to minimize the wasted votes like we found in 2012's election.
This system is good for urban and rural Democrats. It’s good for urban Republicans. It also creates space for independents and third parties.
Washington’s redistricting commission puts power in the hands of political appointees. Here is an idea for you: How about instead we put power in the hands of voters themselves? Yes, let voters decide who represent them—after all these are the folks who pay the taxes and live under the laws and rules passed by government. We can do this with multi-member districts electing candidates with fair proportional voting rules. No constitutional amendment required.