My theory is that Massachusetts voters are more liberal than those in Rhode Island, especially culturally, and thus were more likely to support McGovern, but Rhode Island has more "traditional" Democrats (the state is heavily Catholic, with Catholics that seem more devout than those in Massachusetts) who were more attracted to Carter. Rhode Island is just as Democratic if not slightly moreso than Massachusetts, but seems less liberal.
Well I'll admit I dont know enough about New England to give an informed opinion, but your theory sounds plausible.
Its a similar situation right now in the South. A majority of most states probably self-identifies with republicans but the degree of commitment depends on the extent of their conservatism. Those not so solidly conservative are more likely to defect to the Demorats in a landslide year for them (just as Massachusetts democrats did for Reagan in the 80's)