Let's say your SPL returns 15 results, then the pie chart will show the top 10 values with maximum count and showing the remaining 5 in Other fields.
To match with the event, you can separately execute panel query (which will have all results) and can match count with indexed data.
If you find my solution/debugging steps fruitful, then an upvote would be appreciated
Can you share you search. The 'count' of the other slice will be whatever gets aggregated into the other category. It will show other (x) where x is the number of aggregated rows, but the count value, if you hover of that segment will show you the actual count value.