FEP-9967: Polls update: https://codeberg.org/fediverse/fep/pulls/842
It now contains a section explaining the purpose of votersCount property:
The votersCount property can be used to specify how many people have voted in a poll.
This number will be distinct from the number of votes in case of multiple-choice polls, and can be used to calculate the proportion of participants who voted for a poll option.