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In the states and cycles where a “C” is shown, the margin of error is almost always small. But a “C” is not about the size of the margin of error on its own. It is about the margin of error in relation to the WEP share.
That reflects two things together in those states: the survey had very few white evangelical Protestant respondents, between one and four in each case, and those few respondents stand for only a few percent of the state’s voters.
The margins of error and ranges shown in other visualizations for everyone other than WEP already include whatever uncertainty there is about the WEP share.
For more detail, see the methods section of the article.