Cap rate measures the building. Your mortgage measures you. Confusing the two is how people end up buying a 5% asset with 7% money.
A duplex is listed at $400,000 and the agent tells you it rents for $3,000 a month. That is $36,000 a year, or 9% of the purchase price, which sounds extraordinary right up until you subtract everything that is not rent. The number that survives the subtraction is the cap rate, and it is usually about half of what the listing implies.
β Educational, not financial advice.