A difference would arise if the cash rent is not exactly the same for each month across the lease term. Differences could result from lease incentives/inducements, or there could be a slight increase in rent expense each year to account for inflation. However, to calculate straight-line rent expense, you simply add up total cash rent paid and divide by the lease term in months.
See Also
Deferred Rent: Understanding the Impact of Changes Brought by ASC 842Straight Lease Definition | Law InsiderRent Expense: Definition, How It Works, and Types of CostHow to calculate straight-line rent — AccountingTools
For example, if rent is $50,000 total for a 5 year lease and the first year is free, then you would pay the $50,000 over 4 years at $12,500 per month. However, under U.S. GAAP, you would spread that $50,000 evenly over 5 years, which would make rent expense $10,000 per month.
