"Not Counted Due To Role" in Events Dashboard incorrect

When viewing the Events Dashboard example.org/civicrm/event?reset=1 the breakdown offered for event participants on some events is incorrect, specifically the number given for "Not Counted Due To Role". In my client's site all roles are set to be counted, so I'm assuming therefore that this number should always be zero.

This label and number is also hyperlinked when non-zero. The link provides the search results for all contacts that are participants of the event, regardless of role, exluding only test participant records (which seems correct given that all roles are counted).

In the example that I'm looking at I have 131 participants, 4 of whom have cancelled, and as noted above all roles are set as 'counted', but the events dashboard reports 68 participants as "Not Counted Due To Role". Clicking on that link provides a list of all 131 participant records. I'm at a loss to understnad where the number 68 is being calculated from.

In a second event on the same site "Not Counted Due To Role" reports 1 participant, and clicking on the link provides a list of 9 participants.

I'm wondering whether this might be happening because on this site participants often have multiple roles applied?