diff --git a/mercurial/debugcommands.py b/mercurial/debugcommands.py
--- a/mercurial/debugcommands.py
+++ b/mercurial/debugcommands.py
@@ -1017,23 +1017,33 @@
         common, hds = doit(localrevs, remoterevs)
 
     # compute all statistics
-    common = set(common)
-    rheads = set(hds)
-    lheads = set(repo.heads())
+    heads_common = set(common)
+    heads_remote = set(hds)
+    heads_local = set(repo.heads())
+    # note: they cannot be a local or remote head that is in common and not
+    # itself a head of common.
+    heads_common_local = common & heads_local
+    heads_common_remote = common & heads_remote
+    heads_common_both = common & heads_remote & heads_local
+
+    all = repo.revs(b'all()')
+    common = repo.revs(b'::%ln', common)
+    missing = repo.revs(b'not ::%ld', common)
+    assert len(common) + len(missing) == len(all)
 
     data = {}
     data[b'elapsed'] = t.elapsed
-    data[b'nb-common'] = len(common)
-    data[b'nb-common-head-local'] = len(common & lheads)
-    data[b'nb-common-head-remote'] = len(common & rheads)
-    data[b'nb-common-head-both'] = len(common & rheads & lheads)
-    data[b'nb-head-local'] = len(lheads)
-    data[b'nb-head-local-missing'] = data[b'nb-head-local'] - data[b'nb-common-head-local']
-    data[b'nb-head-remote'] = len(rheads)
-    data[b'nb-head-remote-unknown'] = data[b'nb-head-remote'] - data[b'nb-common-head-remote']
-    data[b'nb-revs'] = len(repo.revs(b'all()'))
-    data[b'nb-revs-common'] = len(repo.revs(b'::%ln', common))
-    data[b'nb-revs-missing'] = data[b'nb-revs'] - data[b'nb-revs-common']
+    data[b'nb-common'] = len(heads_common)
+    data[b'nb-common-head-local'] = len(heads_common_local)
+    data[b'nb-common-head-remote'] = len(heads_common_remote)
+    data[b'nb-common-head-both'] = len(heads_common_both)
+    data[b'nb-head-local'] = len(heads_local)
+    data[b'nb-head-local-missing'] = len(heads_local) - len(heads_common_local)
+    data[b'nb-head-remote'] = len(heads_remote)
+    data[b'nb-head-remote-unknown'] = len(heads_remote) - len(heads_common_remote)
+    data[b'nb-revs'] = len(all)
+    data[b'nb-revs-common'] = len(common)
+    data[b'nb-revs-missing'] = len(missing)
 
     # display discovery summary
     ui.writenoi18n(b"elapsed time:  %(elapsed)f seconds\n" % data)