The Journal of Elections, Public Opinion and Parties (Volume 19, Number 1) back in 2009 had some still very relevant research into how many votes people cast in English local elections where there is more than one vacancy to be filled at once, for example in a three member ward with all the seats up for election at once:
7-15% of total potential votes are unused …
Unused votes occur when electors have a restricted choice of candidates, principally when parties fail to field as many candidates as there an available seats … [and] unused votes stem from a misunderstanding of the electoral procedure … even when parties present a full slate of candidates, some voters (about one in fifteen in London and the districts; about one in nine or ten in the metropolitan boroughs) do not use their full quota …
Low levels of educational attainment among the population at large is also relevant to the explanation.
[Unused Votes in English Local Government Elections: Effects and Explanations by Colin Rallings, Michael Thrasher and Galina Borisyuk]
In other words, a significant minority of people go in to a polling station intending to support party X, but due to a failure to understand how many votes they can cast, they don’t give party X as many votes as they could.
Part of this is down to those running the election, and choices made over issue such as the design of ballot papers and the information provided in polling stations.
However, part of this is in our hands – and how much effort we remember to make to explain multiple voting. An important lesson for get out the vote (GOTV) activity.