diff --git a/internal/controller/mantlebackup_controller_test.go b/internal/controller/mantlebackup_controller_test.go
index 4460ea47..fd31a793 100644
--- a/internal/controller/mantlebackup_controller_test.go
+++ b/internal/controller/mantlebackup_controller_test.go
@@ -513,7 +513,8 @@ var _ = Describe("searchDiffOriginMantleBackup", func() {
 
 	testSecondaryMantleBackups := map[string]*mantlev1.MantleBackup{
 		"test1": basePrimaryBackups[0].DeepCopy(),
-		"test2": basePrimaryBackups[1].DeepCopy(),
+		// "test2" cannot exist on the secondary cluster
+		// because it has a higher snapID than "test0".
 		"test3": basePrimaryBackups[2].DeepCopy(),
 		// "test4" is intentionally omitted.
 	}