First, they don't count all the couples who cohabitate in a marriage like relationship and then break up.
Second, it compares marriage licenses to divorce rates. If a person married and then divorces. That statistically is 50/50. However, if that person then remarries the rate statistically drops to 33%. 2 marriages. 1 divorce. Same person. Is that REALLY a 50% divorce rate? If a person marries 3 times and divorces 3 times, is that a 50% divorce rate? Yes and no.
A statistic of 50% divorce rate more accurately is over 60% - and that is if you do not count marriage-cohabitations without a license.