Explanation:

Activity Explorer logs a DLP rule match event each time any DLP rule condition is met on a file.
File1 matches Rule11 and Rule12 # 2 events
File2 matches Rule21 and Rule22 # 2 events
File3 matches Rule11 and Rule22 # 2 events
Total events in Activity Explorer = 2 + 2 + 2 = 6.
Microsoft notes that Activity explorer shows granular DLP activities such as policy rule matches per item.
The DLP incidents report aggregates by policy match per item, not by each rule in that policy. Multiple rules from the same policy on the same item count as one incident; if different policies match the same item, each policy creates its own incident.
File1: Rules from DLP1 only # 1 incident
File2: Rules from DLP2 only # 1 incident
File3: One rule from DLP1 and one from DLP2 # 2 incidents
Total incidents = 1 + 1 + 2 = 4.
References: Microsoft Purview DLP explains that Activity explorer records detailed DLP activities, while DLP incidents are created when a policy match occurs for an item, and multiple rule matches within a single policy are consolidated into a single incident for that item.