diff --git a/swh/storage/in_memory.py b/swh/storage/in_memory.py
--- a/swh/storage/in_memory.py
+++ b/swh/storage/in_memory.py
@@ -460,6 +460,7 @@
                 'branches': copy.deepcopy(snapshot['branches']),
                 '_sorted_branch_names': sorted(snapshot['branches'])
                 }
+            self._objects[snapshot_id].append(('snapshot', snapshot_id))
         self._origin_visits[visit]['snapshot'] = snapshot_id
 
     def snapshot_get(self, snapshot_id):
@@ -780,6 +781,7 @@
         origin['visits_dates'] = defaultdict(set)
         if key not in self._origins:
             self._origins[key] = origin
+        self._objects[key].append(('origin', key))
         return key
 
     def origin_visit_add(self, origin, date=None, *, ts=None):
@@ -914,7 +916,9 @@
             )
         stats = {key: 0 for key in keys}
         stats.update(collections.Counter(
-            obj_type for (obj_type, obj_id) in self._objects.values()))
+            obj_type
+            for (obj_type, obj_id)
+            in itertools.chain(*self._objects.values())))
         return stats
 
     def refresh_stat_counters(self):
diff --git a/swh/storage/tests/test_storage.py b/swh/storage/tests/test_storage.py
--- a/swh/storage/tests/test_storage.py
+++ b/swh/storage/tests/test_storage.py
@@ -1414,12 +1414,45 @@
         expected_keys = ['content', 'directory', 'directory_entry_dir',
                          'origin', 'person', 'revision']
 
+        # Initially, all counters are 0
+
         self.storage.refresh_stat_counters()
+        counters = self.storage.stat_counters()
+        self.assertTrue(set(expected_keys) <= set(counters))
+        for key in expected_keys:
+            self.assertEqual(counters[key], 0)
+
+        # Add a content. Only the content counter should increase.
+
+        self.storage.content_add([self.cont])
 
+        self.storage.refresh_stat_counters()
         counters = self.storage.stat_counters()
 
         self.assertTrue(set(expected_keys) <= set(counters))
-        self.assertIsInstance(counters[expected_keys[0]], int)
+        for key in expected_keys:
+            if key != 'content':
+                self.assertEqual(counters[key], 0)
+        self.assertEqual(counters['content'], 1)
+
+        # Add other objects. Check their counter increased as well.
+
+        origin_id = self.storage.origin_add_one(self.origin2)
+        origin_visit1 = self.storage.origin_visit_add(
+            origin_id,
+            date=self.date_visit2)
+        self.storage.snapshot_add(origin_id, origin_visit1['visit'],
+                                  self.snapshot)
+        self.storage.directory_add([self.dir])
+        self.storage.revision_add([self.revision])
+
+        self.storage.refresh_stat_counters()
+        counters = self.storage.stat_counters()
+        self.assertEqual(counters['content'], 1)
+        self.assertEqual(counters['directory'], 1)
+        self.assertEqual(counters['snapshot'], 1)
+        self.assertEqual(counters['origin'], 1)
+        self.assertEqual(counters['revision'], 1)
 
     def test_content_find_with_present_content(self):
         # 1. with something to find