(1) Account for vacancy rates (e.g. 10% of the time). (2) Management fee is a % of monthly income, a flat fee when vacant. There’s fees for finding a new tenant. (3) Yearly income is expect to grow at a given % which isn’t not necessarily equal to appreciation (4) Jurisdictions: some have rent control. Some rent control resets between tenants, others are fixed for the unit’s lifetime. Rent control limits yearly income growth by a % (loosely related to inflation). (5) Maintenance funds. (6) Turnover expenses: in addition to fee to find new tenants, turnover often involves repairs and improvements.