You can see these calculators in many places, and basically what they are is predictors based on actuarial tables (Such as the ones insurance companies use to set premiums), compared against average life expectancy.
Basically, it should take into account all your lifestyle and risk factors, and apply those to you and the table, and produce an estimate of your "age". So, if you are 25, male and an overweight smoker, your estimated age will be more than a 25 y/o male who works out and eats well.
Some of the more elaborate ones will also take into account heredity and other extrinsic factors, but they are all a guess. We've all heard about the healthy runners that drop dead at 40 (Fixx for example), and the whiskey drinking, lard eating gent that lived to 105. It is, however, a good reminder that our choices now can have a large impact in later life.
To get the ages the same, it's hard to say since we have no idea what your family history is, or what other factors you put into the test. I can say that obesity, smoking, more than moderate drinking and lack of exercise are some of the bigger factors that will make your body age higher than your actual age.