This is because an at-large system isn't the same as a proportional representation system. In an at-large system, 51 percent of voters can still control 100 percent of the seats. This is because each voter gets one vote per seat. For example, if a hypothetical state had 5 Representatives elected on at at-large basis, and 51% of the state favored one party, they could win 51% of the vote for each seat, and win every seat.