Analyzing a property consists of no less than a conglomeration of 30 to 50 numbers that need to be entered onto some sort of paper form, spreadsheet, or database that can do the calculations and every calculation can make a significant difference in regards to whether or not a prospective property will be profitable. Leaving out one or more of the calculations makes a significant difference and without a few of the calculations you may end up buying a property that loses money, or you could not buy a property that is a goldmine.
The next serious problem and mistake I see investors doing is they refuse to enter realistic and true numbers in their calculations so their analysis is skewed and not accurate. For example, there is a huge difference between an annual maintenance rate and the cost to clean and put a rental unit together every time a tenant moves. So, for my calculations, I may use something like $1500 per year for repairs like a water heater, furnace, roof, plumbing, exterior painting and maintenance and then I put into my calculations the cost to clean, paint and put a rental unit back together every 5 years when the average tenant moves and that cost is $5,000 to $8,000 depending on the size of the unit, whether it is a house, or an apartment and the size of the yard and number of plants and trees.
The next most critical numbers that have to be considered is the principal pay down when you have a loan, tax depreciation over a number of years, some sort of inflation rate, annual rent increases are very critical and my favorite calculation that makes breaks many deals is property taxes and annual increases. I always stretch my numbers out for 10 years and if my annual return is less than 15% I scrap it.
The next most critical thing investors need is a good business model. If you don't have a business model to follow then you are shooting in the dark. My business model is to double my investment capital every 1 to 2 years, or to earn 50% to 100% on my investment capital every 1 to 2 years. If you don't have this business model then most-likely you will not achieve this goal with the exception of being very lucky.
While shooting for a return of 50% to 100% every year sounds unrealistic it is actually quite simple. Suppose, you have $50,000 to invest. All you need to look for is a property you can purchase with $50,000 down and then, using simple math, you only need to get that property for $25,000 less that what it is worth and you just earned $25,000, or you earned a return of 50% on your investment capital the day you close escrow, or if you purchase the property for $25,000 less than its value, increase the value by $25,000 by doing a little cosmetic work and increase the rent you could easily earn 100% or 150% on your money in less than 1 years. Again, if you don't use such a business model you will not achieve that goal.
So, to answer your questions in regards to why you are crunching numbers and they never make sense, I would love to see the numbers you have been crunching and then I will present a spreadsheet that does all the calculations I mentioned and maybe some BP members have some good calculations to add to the spreadsheet.