In our current system, we elect individuals who might or might not be identified as being members of some political party. The party affiliation on the ballot is merely there to help voters in recognizing the candidates.
Despite the party affiliation that might be on the ballot for any given candidate, at the end of the day it is the individual who is elected, the individual who gets sworn-in and takes on the responsibilities of the job, and it is the individual who gets the paycheque from the taxpayers, not the party.
While, to be sure, many people do vote on the basis of the party with which a given candidate is affiliated or perhaps on the leadership of that party, it is not always so.
Some people do vote for the individual, regardless of, or perhaps in spite of, his or her party, some according to a specific issue or policy, and others based on the candidate’s gender, or religion, or even ethnic background or race, whether we like the idea or not, or for any number of reasons. It’s the voter’s vote, and he or she is entitled to use it on whatever basis he or she chooses.
It’s specious and problematic, then, to say that votes registered to given candidates are really votes for the party, and then, based on that, that the given party “should” have received some given percentage of victories.
We could just as easily, and just as validly, interpret the vote in terms of the genders of the respective candidates and demand that the resultant gender ratio be recognized in the seat count, or by religion, or ethnic or national origin, gender orientation, or… whatever.
Until or unless we institute a voting system in which we actually cast a vote directly for the party (or some other given aspect), it will always be problematic to say that “the party” (or any other given aspect) received some given proportion of the vote.
Yet, it is just such faulty derivation that gives rise in many to the clamour to “fix” the system with proportional representation.