The increased temperatures may be because a temp of 120°F is just right for extremely rapid bacterial growth. 150°F is a perfect holding temperature, although it will cook. The newer models with higher temps are not as likely to develop bacterial growth in the food as it cooks.
Some models of crockpots, according to my old "Crockery Cookery" cookbook, will actually exceed the boiling point. To measure how hot a crockpot gets, you might want to use some oil instead of water.