Let's say you have a multiple-select with 3 choices, and 20 respondents answer your survey, and each one selects 2 of the 3 items when they answer it.
That means you have 20 respondents, but 40 choices that were selected (20 people x 2 choices each).
% of choices will be calculated based on the #selected for each item/40 selections that were made. In this case, the % will all add up to 100 because they are based on selections, not people.
% of respondents will report the results based on the # selected for each item/20 responses - or "What % of the respondents selected this choice?" In this case, the total % will add up to more than 100% because each person could (and did) select more than one item).
We find that % respondents is how our teams often want to see multiple selections in terms of the % people selecting that answer vs the % of the selections each answer represents, but your use case may vary.

https://community.qualtrics.com/XMcommunity/discussion/comment/46468#Comment_46468Thank you so much for the explanation!