Typically this indicates a multiple choice question, e.g. Choose the languages you are comfortable programming in.

Then the percentage indicates the number of respondents who selected that / the total number of responses.

Multi answer questions in the survey may explain that?

I see. One way of calculating % in multiple choice is ((total No. of C likers)/ total count of answers) * 100 to get % of popularity of C.

You can just add all the percentages and divide by that number and you'll get the number you're talking about.

Respondents are allowed to choose multiple answers for each question, so the totals != 100%.