One participant record exists for each event. If one query groups participants for Event A and another query groups participants for Event B, each query has unique records. If the same constituent attends both events, that constituent has separate participant records for each event. Merging these queries using AND or SUB gives inaccurate results.
Also, merging Participant or Event queries with operator SUB when filtering on Constituent fields, such as Constituency Code, can result in inaccurate results.