The canonical sequence of aperture values is 1, 1.4, 2, 2.8, 4, 5.6, etc. However, every other one of those values beginning with 1.4 is an approximation. 1.4, for example, is an approximation for the square root of 2, or 1.414214.... (The other values, 1, 2, 4, etc., are exact, since they're even powers of the square root of 2.)
This fact is just trivia most of the time, but when doing calculations with f numbers, it makes sense to use the most accurate values possible. Therefore, when you pick an aperture value from one of the program's drop-down lists, it will silently use the accurate version, though it will usually only display the approximation, for simplicity's sake.
If you want to see what values f/Calc is using for these aperture values, you can use the f Number page: put 1 in the stops field, then scroll through the f1 field with the rounding turned off.
Note that if you type in one of the common approximations, like 5.6, f/Calc will not second-guess you and substitute 5.656854.... It will assume you really want to use the value 5.6. This behavior lets you see how the difference between the approximate f number and the accurate version changes your results.