Why can a person win the popular vote in a presidential election and lose the electoral vote?

There are actually 51 elections for president. One in each state and one in DC. If a candidate wins some states by a small margin, it will counteract a big win (for the other candidate) in fewer states. The result could be that one candidate could win the popular vote but lose the electoral vote. Most states have a winner take all election while some divide the electors by the ratio of votes.