If you choose (in System Options) Daily accruals, or Monthly accruals without a 360-day year, the accrual of rent for a lease paid quarterly, semiannually, or yearly is calculated evenly for each day of the rent period.
Since some months are longer than others, the accrued rent for each month differs. If you choose the System Option “360 day year for rent and interest accruals,” each month will be treated as having the same number of days, and the rent accrual for each month will be the same.