diff --git a/tests/domain/sap10_calculator/test_real_cert_sap_accuracy.py b/tests/domain/sap10_calculator/test_real_cert_sap_accuracy.py
index d19acf630..24dde9482 100644
--- a/tests/domain/sap10_calculator/test_real_cert_sap_accuracy.py
+++ b/tests/domain/sap10_calculator/test_real_cert_sap_accuracy.py
@@ -898,9 +898,21 @@ _EXPECTATIONS: Final[tuple[RealCertExpectation, ...]] = (
     # note a few lines below: applying Table 11 unconditionally regressed
     # the full 1000-cert corpus MAE by +0.16 (lodged software doesn't
     # apply Elmhurst's blanket convention). Verified: forcing Table 11 on
-    # for just this cert moves 76.33 -> 73.62, ~62% of the 76.33-vs-72 gap
-    # — confirms this is the dominant driver, not a mapping/entry error.
-    # PINNED to the observed exact match (vs lodged, the correct target).
+    # for just this cert moves 76.33 -> 73.62, ~62% of the 76.33-vs-72 gap.
+    # The remaining ~1.6 pts is a second, separate cause: (6) Solar gains —
+    # the BUILD entered all 12 windows as one 15.32 m2 "North" group
+    # (E.set_single_window), but the cert's real sap_windows lodge 3
+    # orientations (NW 7.05 m2, NE 1.21 m2, SE 7.06 m2 incl. the largest
+    # single window) — collapsing to due-north understates solar gain
+    # (Elmhurst Jan (83)=56.9 W vs our real-orientation 123.4 W). Verified:
+    # forcing our engine to ALSO use a single synthetic North window (i.e.
+    # replicating the build's simplification, not a cert or engine fact)
+    # plus the forced Table 11 above reproduces 72.62 -- matches Elmhurst's
+    # 72 almost exactly. Conclusion: the calculator is faithful (it already
+    # matches lodged exactly using the cert's REAL window orientations);
+    # both residuals are Elmhurst-worksheet-reconciliation artifacts (a
+    # deliberate spec choice + a build-entry simplification), not mapping
+    # or calculator bugs. PINNED to the observed exact match vs lodged.
     RealCertExpectation(
         schema="RdSAP-Schema-21.0.1",
         sample="uprn_100061275133",